@kytta You might try looking in logs inside the modem, if you can get in the modem and it has logs. I discovered that my cable internet connection from my ISP was incorrectly attenuated by an F cap on the end of an exposed coaxial splitter. I removed it and my internet connection stopped dropping once or twice per day like yours. Wild guess?
@blucrunch I don't have coax, but DSL, and I don't think it's the issue, since the connection still works over the Ethernet. But thanks for the guess!