Roku Ethernet Wired Connection to xfinity Interent fails after modem replaced

This has been discussed on other posts, I have read them all.

We have been having intermittent connection issues with our xfinity internet service, usually every day or two and the only way to restore the connection is to reboot the modem. Using xfinity Assistant, the signal to our modem is fine. Our existing modem is a Motorola MG7540 DOCSIS 3.0; we use a tp-link DECO 9+ mesh for wifi.

I decided to replace the modem which was a number of years old. I tried a number of DOCSIS 3.1 modems and each time after activation our Roku Ultra’s with wired Ethernet connections would not connect to the internet. I returned those and dropped back to a new Arris Surfboard SB6183 DOCSIS 3.0 modem.

It established a connection to the internet fine with download speeds approaching 600 mbps. The Roku Ultras will not connect to the internet with the wired connection. When testing the connection, they make the wired connection but will not connect to the internet. Lights on each Roku flash at the port but the operating system cannot establish a connection. When I plug the Ethernet cable into another device, it works fine. Not being able to get the connection to work, I switched back to the older modem and reactivated it and each time the Roku Ultras work fine and make the needed internet connection.

I’ve been through all of the troubleshooting tips including re-installing the Roku OS on each device, rebooting our network routers and such, rebooting the modem numerous times, I used the Roku Secret Screen to Enable Network Pings, all to no avail.

I would like to replace the modem but do not want to purchase a new streaming device. The Ultras work great.

I am looking for suggestions on what I can do next.