Can't sync WhatsApp messages properly.

Don't have an iCloud backup.
What might be this particular privacy setting preventing the synchronization?

Contacting WhatsApp Support is just a waste of time!

Proton VPN. Though the problem had persisted after I deleted it for the first time, so I decided reinstalling it as I thought it probably had nothing to do with this issue. But now when I uninstalled Proton VPN once again, WhatsApp suddenly began working properly on my phone.

What's going on!

I reinstalled and uninstalled Proton VPN once again, and the issue's gone. So I'm now thinking of the following possibility: each couple of times I reinstall and uninstall it, the problem will emerge and be gone. Reinstalling and uninstalling the app for the first time won't do anything at all, but repeating the very same steps once more will make WhatsApp and the Internet connection work as expected. Having to trace the issue like a dev or software engineer or some detective is definitely not the most pleasing thing given I have to check my WhatsApp messages every so often and they include work-related stuff, but having to waste time trying to reach out to the WhatsApp support team is a much greater challenge!

I'd been deleting the app itself and keeping its documents, thinking I'd reinstall it later sometine and the configuration profile alone wouldn't cause any issues since the app was deleted, but finally decided to delete the VPN configuration profile as well, and the issue appears to be gone for now. The thing is, the client and all the servers are shown to be operational at status.proton.me. The following attracted my attention though:
https://status.proton.me/incidents/dzl872r2tn5m
https://status.proton.me/incidents/3lnypxtj4krd

Whenever I thought the problem might've nothing to do with Proton VPN, I would reinstall it, only to have the problem back; and whenever I reinstalled and launched it, I would just get stuck on the main screen. Since it's a VPN client, it requires an active Internet connection to connect to whatever server you pick. And since it somehow messes around with my connection, it makes itself fail to load properly, assuming it's this very app what causes the problem. This is why I can't even get past the first screen where I can't focus the VoiceOver cursor on anything except for the status bar.

I use it because it's free and accessible. As I said earlier, I don't think it's supposed to somehow prevent me from using WhatsApp while still letting me use all the other apps. Though I did have issues with other apps like Youtube and LinkedIn, but WhatsApp would not connect to the server and fetch/sync the messages no matter I did. I had been using WhatsApp for the several months during which it had coexisted peacefully with Proton VPN. I studied Political Science and International Relations and am currently doing a master's degree in International Relations, but no theory can explain why this war culminating in Proton VPN being totally crushed broke out between WhatsApp and Proton VPN. :)

This question raises another one: Why and how could I use WhatsApp until this issue appeared out of thin air?
Yes, I'm not a fervent fan of Mr. Zuckerberg and have looked for other alternatives, but these apps are intended to let their users chat with others, after all, and thus require those you wish to chat with to use them as well, so migrating to some other app has never become something I managed to do, even though I also use Telegram and even have Signal and BiP, an app maintained by Turkish developers, on my device. I will have to remain in and keep following certain groups in any case, so can't quit using WhatsApp. Using a VPN service is also quite essential to me not only for privacy but also because I may have to visit certain websites so as to access stuff blocked by the authorities.

Sure, three months is better and greater than nothing, no? So let me thank for you the suggestion as I also clarify something: It's not really like you can't access anything in Turkiye as in some countries like China or North Korea. We don't have such heavy surveillance but certain things are blocked due partly to rising tensions with the US and certain other countries. The Turkish government does that on the basis that certain websites host misleading stuff made (up) by certain groups backed by the US.

Okay, Proton advertises itself as a crowdfunded company founded by scientists working at CERN and making its apps free for everyone, claiming it's their very purpose to fight surveillance and enable everyone, including activists, journalists, those subject to government surveillance and everyone believing in the unconditional right to privacy, to have access to free VPN services, while also offering paid premium plans. So it's not a company dependent on and/or supported by countries like Russia or China that might probably be more likely to be considered against such apps as WhatsApp. And as I said, I had been using WhatsApp for several months up until this mysterious thing happened for no reason known to me.

Well, all I know is that WhatsApp ceased working with Proton VPN or vice versa, and as you said, I probably need another alternative.

You must've had a similar experience when realizing an update breaks an app's accessibility or even the core functions. So feeling that way is unfortunately more than familiar to me.