Looking for some help trying to access my Motorola MB8600 cable modem admistration page from behind my Archer router.   I see a lot of folks are having this issue but not seen any solutions or explinations about what is going on.

The modem's admin page is at 192.168.100.1  and is accessable by directly connecting the PC to the modem but not on the LAN side of my router.  The connection times out.  Pings to 192.168.100,1 also fails.     My lan is setup for 192.168.0.1 with a mask of 255.255.255.0

Setup:  Comcast cable is connected to the modem,  modem's ethernet port is connected to the WAN port on the router and my computeris connected to the lan side of the router.  Nothing fancy, no other devices involved.  The internet connection is working fine I just can't figure out how to access the modems adminstration page.  I am not all the network literate so be kind.

I did find one suggestion that appeared to work but I don't understand why.  The suggestion was to change my routers default ip address from 192.168.0.1 to  192.168.1.1.  ( default gateway ??)  After doing this the modem showed up but it broke all my DHCP address reservations for my IOT devices. I have no clue of why this allowed access to the modem and sinced switched it back so my system works again.

Any help or explinstion would be appreciated