The ISP can cause many issues. I have had issues with signing on to my Archer C8 using http://www.tplinklogin.net and other times 192.168.0.1. One or the other would always get me signed on or the...

It may be your PC that is putting it to sleep as part of the energy savings mode as it is just another HHD. Check your PC settings.

Would something like Dashlane or LastPass (password managers) do an auto login from your browser?

Connect a PC direct to the modem with an Ethernet cable bypassing the router. See if you get the same issue. If so the issue is with the modem or your ISP. If not, try a factory reset and reenter...

Need to isolate the issue a bit. Is it on the wired connection or on WIFI and if WIFI, is it 2.4 or GHz band?

May be channel interference so try a different WIFI channel and channel width. Also be sure the other router is not on if the SSIDs are the same.