do many routers not support https for their config pages? if so, that's terrible on their part, get a new router. If they do, then accept the cert in your browser and move on.
If this trend continues it means you will no longer be able to configure these devices with a webbrowser but forced to use the manufacturers "cloud solution" or install an app where both ends of the TLS connection can be controlled and you're not bound to public CAs.
It’s not a good trend, but this is a plus for most consumers who don’t care about local network security.