VMG8924-B30A acting wierd even after factory reset

Maneren

I have a VMG8924-B30A router (or is it a modem?) for a little over 2 years now, bought from an ISP. I have made quite a lot of changes to settings like NAT, DHCP, etc. and it was working just fine for those 2 years. Recently I wanted to do a little adjustment to the settings, but couldn't sign in to the router (using the web interface). I was like, ok, I have the wrong credentials (which is weird since I use password manager, but it is possible), but after trying all possible passwords including factory one, I resigned and decided to reset the router to factory settings (it took holding down the reset button for like a minute in order to do that, anything shorted just rebooted it while keeping the settings).
However, and this is the main problem, now I can't sign in with default credentials either. After I input them it reloads the page without any response (such as "wrong password", etc.). So now I am stuck with an unconfigured router, unable to change any setting.
And, on top of that, it started to act very weirdly, sometimes it just disconnects wifi from the internet while the ethernet still works, sometimes it disconnects from the internet completely, etc. Thankfully it takes just a reboot to fix it (for now), but it is very annoying nonetheless.
I tried to reset it one more time with no signs of it getting better.