If you have Spectrum and got sent a new Modem and Router this is how I fixed it. I fixed it by getting my own router. I got a Netgear Dual Band. The dual band means you can use 2.4 GHz or 5 GHz. The new Spectrum was forcing it to use 5Ghz. I think this is what did it. Or it might have been something else but it works now. You might be able to use any stand alone router but the one I got was Netgear AC1750 WiFi Router. It was about $130 which sucks but like I said... it worked.
I can’t promise it will work for you but just wanted to share this!