Cannot Download iOS Updates Using Cellular Data

Check your settings as follows:
1. Open Settings, Cellular, Cellular Data Options.
2. In here you have the option of enabling Roaming, but if you scroll down a bit, you'll come to an option called Data Mode.
3. This option gives you three choices typically, 5G auto, standard, and low data mode. Standard is ideal if you are connected to Wi-Fi most of the time, as it will prefer Wi-Fi over your data. 5G auto will prefer 5G over Wi-Fi, regardless if you're connected or not. Low data mode is just as it sounds, it bottlenecks your data, preventing you from doing things like downloading updates etc.

HTH.

I have completely cancelled my fixed Internet service, am fully on unlimited 5G cellular with a 668mbps downstream bandwidth, 45mbps upstream bandwidth, and 15ms average ping with a 2ms jitter just tested at home, which is a lot better than the fixed DOCSIS Internet connection that I was on previously, and while I don't experience this on the iPhone providing the Internet connection itself, I do experience it on all other Apple devices connected to it through Personal Hotspot, with the notable exception of Macs, which makes me believe that this is an iOS bug. The symptoms in my case are the inability to update iOS and iPadOS or make iCloud backups on all devices except the aforementioned Macs and the iPhone providing the Internet connection, and all metered connection protections are disabled on all the affected devices.

Since my life is a chaotic mess right now, I haven't found the time to reconnect my Raspberry Pis and Wii-Fi 6 access point back to my network yet, and still haven't set up a 5G Internet connection on my iPad, which will eventually replace my iPhone as my Internet router. However this thread made me curious and will actually test downloading something big using a Raspberry Pi connected to my iPhone's Personal Hotspot to check whether it really is an Apple problem.

I did not think to ask above, but is this an iOS 26 thing, or an iOS issue, in general?

My problems with firmware downloads and iCloud backups over Personal Hotspot are all on devices running iOS 26.0 or 26.1. Some of those devices were eventually upgraded using one of my Macs over exactly the same Personal Hotspot connection, and to rule out the possibility of it just working on macOS over a wired USB-c connection, earlier today I upgraded a Lightning iPhone that was having trouble downloading iOS 26.1 itself using this Mac to download the firmware over Wi-Fi Personal Hotspot instead. I may still have an iPad on iPadOS 18 that I can try upgrading over the same Personal Hotspot connection if needed, and already have one of my Raspberry Pis here ready to download a few gigabytes on Linux over the Wi-Fi Personal Hotspot connection to test this further,, but I'm growing increasingly more convinced that this is truly a problem with iOS itself, and am going through a laziness moment because in order to connect to the Raspberry Pi I have to configure a DHCP service on my Mac and feel kinda sleepy, so I guess that I'll just head to bed and report back later.

I'll have to see how things look on my old backup iPhone, once 26.2 is released publicly. In the meantime, my primary device will remain on 18.7, unless for some reason Apple releases another iOS 18 update. I am hopeful that some of the one off bugs will be fixed in 26.2, such as the email scrolling issue, for the report on dark mode having weird and random bright spots on screen while active. As someone who unfortunately has to deal with severe photophobia, I can do without that latter issue, I suppose there is always screen curtain...
I guess I will see how things are looking next week.

After waking up from my nap I picked up the aforementioned iPad on an old iOS version, which as I found out had absolutely nothing on it since apparently I just reset it to factory defaults at some point, and then ran through the setup process in order to verify whether the problem manifests there. Unfortunately I intended to use my original Apple Account on that iPad but couldn't remember its password so I went to reset it on an iPhone that is already logged in to that account and that also failed with an error message stating that it couldn't connect to Apple's servers again over Personal Hotspot, so I ended up abandoning the iPad setup and going for the Raspberry Pi.

The Raspberry Pi is physically connected to my Mac Studio's Ethernet port, on which I'm also running dnsmasq to provide it with a DHCP server so that it can actually get an IP address thus avoiding having to use a serial debug probe to connect to its console. There's no routing between my Mac Studio's Ethernet port and the USB network device created by my iPhone to provide Internet to my Mac, so to access the Internet from the Raspberry Pi I had to configure it to connect to the Personal Hotspot shared by my iPhone via Wi-Fi, and then downloaded a firmware directly from Apple's content delivery network straight to /dev/null to avoid slowdowns caused by the performance of the Micro-SD card that I use for storage on the Pi, without any issues as shown in the following Terminal output:

jps@pi-hole:~ $ curl https://updates.cdn-apple.com/2025FallFCS/fullrestores/089-14505/988A96BB-5554-4E75-A32B-C79031B36B7C/iPhone12,8_26.1_23B85_Restore.ipsw > /dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 9032M  100 9032M    0     0   9.9M      0  0:15:11  0:15:11 --:--:-- 13.1M

Notice that cdn-apple.com although strange is indeed an Apple domain according to its name-server records, and Apple's authoritative name servers do respond to queries about that domain so the download location is legit.

In conclusion, the fact that I managed to successfully download a single file with roughly 10GB from Apple's content delivery network on both a Mac running macOS and a Raspberry Pi running Linux, is proof that the problem is highly likely to be an iOS bug. There's one last thing that I can try, which is to put my Wi-Fi 6 access point in the middle between my iPhone's Personal Hotspot and all my other Apple devices, which should be enough to hide the fact that I'm accessing the Internet from a shared Personal Hotspot connection from the iOS devices, and test how those devices behave under those conditions, but I won't do that today.

With that final test put aside for now, are you saying that iOS is currently unable to download iOS updates, whether it be from Wi-Fi, hotspot, or cellular data? Just making sure I have this right.

Yes, both updates and backups stall. Updates stall at Update Requested, and iCloud backups stall at 2% for a while and then fail with a message alleging bad network conditions, but this only happens on devices connected through a Personal Hotspot provided by another device; the device with the cellular connection providing the Personal Hotspot works perfectly fine entirely over cellular.

If the limit IP tracking feature is the root cause, then I'm at least glad you figure that out. I seem to recall a previous version of iOS, where limit IP tracking was causing network issues as well. I want to say iOS 17, but not certain.
If it's an issue with hotspot sharing, as @João Santos suggests, then that's gonna cause issues for a lot of people me thinks.

Then I guess that we're talking about different things. Limit Address Tracking and Private Relay were two things that I disabled since carriers often use Carrier Grade Network Address translation, which causes problems with that functionality, but that did not fix the problems that I'm experiencing with all iOS-derivative devices inside a Personal Hotspot network failing to connect to Apple's servers. The iPhone that is signed in to my original Apple Account isn't even able to use Private Relay since I'm not subscribed to anything from Apple on that account, and to make sure I also ensured that those features were truly disabled on my current iPad I tried to make an iCloud backup and it stalled at 2% for a while again until giving up and blaming the problem on network conditions.

Well, I was hoping for a simple fix. That's what I get for hoping for a miracle, I suppose. Limit IP Tracking is definitely not the issue in your situation, and probably not going to be the situation for most other users experiencing this. For me, it's waiting until next week and testing out iOS 26.2 on my old backup iPhone. Since that device absolutely needs to connect to hotspot, or a Wi-Fi, as it has no data of its own.
I'll report back here once I have 26.2 installed, unless you find something out sooner.