Did you t ry 192.168.100.1?

Something else to keep in mind is that even through the t25 has 2x ethernet ports, only 1 device can be connected to the ethernet through xifnity. They only support 1 public ip address sadly. 

I have tried 192.168.100.1 - as well as 192.168.0.1 and 192.168.1.1 - nothing (connection times out)

I also know that only 1 connection is supported - the 2nd port is a bridged port unless explicitly setup otherwise.

When I has the SB8200 connected, and running for over a year, I had zero problems getting to the ‘management page’ of the device - now after the T25 is connected and running, and adjusting the DHCP of my router to the 192.168.100.x range, I cannot connect to my modem. DHCP range starts at 192.168.100.100 and goes up, but that really shouldn’t matter should it?

Your dhcp range for the router shouldn’t be in the same subnet as the modem. 

I’d change that back to either the 192.168.1.**** or 10.0.0.******

You can always try disconnecting the router from the modem, powering the modem down for a couple minutes, and then powering it back up with connecting a pc to it to see if you can access it. 

DHCP range - it was the only thing I could think of to see if it would work.

UPDATE - So I thought ‘why not connect directly to the modem to see if it works that way’ and it did - with the 192.168.100.1 IP - so i reconnect the router and give it a minute; router has no connection - like there is no connect at all… all the lights are on, on the modem… so i pull the plug on the modem and force a restart, and everything works, except trying to access the WebManager of the modem….

All I am trying to figure out is how to ‘route’ in the router to access the modem WebManager. I had no problems doing that with my SB8200...