AC1750 / SB6190 - "Cables not connected"

[MN_Traveller]

Installed a new SB6190 modem with an AC1750 router.

Installed the modem. contacted Suddenlink, activated the modem, verified the connection. (Including access of the modem's web page.)

Moved the yellow cable from the computer to the AC1750. Using an Samsung S10, attempted to configure the router using the Android app.

S10 connects. Five minutes later, the screen says "Cables not connected." Swapped ethernet cables. No improvement.

Connected the computer to the AC1750. Am able to connect to the AC1750. Unable to open the modem's web page. (192.168.100.1)

Any help is appreciated.

Thanks
MN