@David-TP 

Hi David, thanks for your fast reply.

I bought the MR200 second hand about a week ago, the previous owner bought it but replaced it with a 5G capable model after a couple of weeks.
For the first couple of days (2 or 3) the MR200 worked just find, then all of a sudden the web interface started to behave like this, I logged in a couple of times per day to keep track on the 4G data usage.

I could not use the Tether app since I couldn't access the WiFi after the factory reset and couldn't configure it via the web interface, I did connect my computer via Ethernet and configured the WiFi via CLI, that worked fine, and the Tether app works on my phone.

Now when I test the web interface it's "almost working", as you can see in the screenshots below the status is as follows:

The Basic tab shows the Network map correctly, everything is there like the WAN IP, type of mobile connection etc.
I can not change the WiFi settings, either the save buttons don't do anything, or I get sent back to the Basic Network map.
If I click on "TP-Link Cloud" the page is empty, and if I hover above the "i" I only see a small empty box.

The Advanced tab loads more than before, but there is no data in the fields, even though the router is up and running.

I also noticed something strange, when I connect to the new WiFi I set via the CLI my Windows 11 computer says the WiFi is not connected to the internet, even though internet works just fine. 

Is it possible to do a factory reset via the CLI, and would that make any difference to the one I've done by holding the Reset button on the router itself?

 Edit: Windows 11 reports no internet connection when my computer is connected both via Ethernet and WiFi, even though it does work, or at least http traffic does since that's what I'm using as I'm posting this.