Adguard is dodgy if you leave it alone or select too many protections. Even on pc and mobile it locks up now and then but if you force it to stop running, clear it's cache and restart it then it should work again. On routers it's probably best just to leave the basic functions selected because the more you have the more likely that Adguard will misbehave.
First of all, I managed to update to 4.7.4. I disabled repeater mode, disabled vpn to get the router as minimal as possible. After that I tried upgrading and this fixed it. Now I am on 4.7.4
So after the upgrade I tried enabling AdGuard again, let it update all lists and after that fully enabled AdGuard, and now everything is working as expected again.
The only thing is, I still don’t know what caused the issues, but I will monitor it.