I just bought this modem (SB6190) and am already at my wit’s end trying to get it to work with my router! I was able to activate the modem myself via the Xfinity app no problem, and it works just fine when I connect the modem directly to my computer via LAN.
HOWEVER, the instant I try to connect the modem to my router, it simply will not work! My router’s light indicates that it isn’t connected to the internet, and when I try to configure the router via LAN connection, it says it can’t find its assigned IP address. I’ve power cycled both the modem and the router several times in the correct order (i.e., power both off, power modem on, wait until connected to Xfinity, power router on), but the problem persists.
To make sure this was a modem problem and not a router problem, I re-activated my Xfinity Gateway and plugged it into my router into that. And hey, what do you know—everything worked flawlessly!
So, can someone PLEASE help me figure out how to get this thing to work before I go insane?
Many thanks for any help! 
