@djf5502 

I have the same issue with my Archer C8. My modem is a Motorola MB8600 connected to Xfinity using Apple PCs.

I do not remember having the issue with different PCs, routers, and modems but the router address may have been different.

Like you, no big deal as I can bypass the router if I need sign on to the modem.

djf5502 wrote

I normally change the router address to 192.168.1.1, but decided to reset my router to original defaults and leave it at 192.168.0.1.    As you would expect everything works fine, except I noticed  I've lost the ability to connect to my modem that has a default address of 192.168.100.1.

@djf5502 

I'm not sure if you did or did not connect to the modem before changing the IP Address of the router?

It also might be the modem not responding as well? Did you REBOOT both the modem and router after changing the IP Address of the router? Might make a difference?

Also, if you have a Security Suite, have you made the LAN a TRUSTED LAN? Varies by Suite how to do that? Might be other places too that need changing?

Of course, depending on ISP and the specific modem, did you ever have access to the modem you have now? I've got Spectrum (Charter) and a majority of the modems they provide are GUI locked... can't get to it.