Used to work independent repair and we’d fairly regularly get people going “uhhh just don’t look at XYZ 👀😰” haha. Can confirm none of us care. It’s part of the job. Pointing it out will just make us look to see if it’s something funny or not. OP’s lucky it was just his DNS history heh.
You likely can’t connect a third party router directly to the ONT (it’s not just a media converter btw), usually requires the modem to authenticate with PPPoE or similar, some ISPs are willing to help with custom setups but most will tell you it’s “unsupported” because it leads to headaches for them when something breaks. Sounds like your devices might be connecting to the old router - is it still turned on? Would recommend unplugging the old one entirely for now. If you’re still having problems it’s time to call the new ISP for support, might be an issue with their equipment or your account on their end.
Is this in your attic? Some places get intolerably hot/cold in attics, leads to a lot of dead electronics.