Can’t access routerlogin nor have account info

Apparently everything in the apartment has been logged into the guest account, since we moved 16 months ago. I now have to connect a new device, and it doesn’t detect the guest account. I’ve tried numerous times to load the routerlogin site, with no success. I downloaded the app again (newer phone) and my router doesn’t have a QR code on it, and I can’t find where to use the serial code as a form of login. And to top it off… I don’t remember the password to the main account on the router. Where do I go from here???

TIA