Your ISP's DHCP is not functioning properly
I am having this same issue I've seen others have. I can plug directly into the modem and have good connectivity with reliable speeds, but as soon as I try to use any router, the router will get no connectivity. I've replaced my router thinking it had just crapped out. I've talked to tech support and all they want to do is send a tech out to test signals. I need to try to get this resolved asap as my kids are supposed to be doing remote learning. Anyone's help would be tremendously helpful.