Arris Port forwarding

I have installed an Arris SBG7400-AC2 to replace the rental cable modem. The modem has been set up through the ISP and everything is working. However, I can’t get the port forwarding for remote viewing to work. Tablo says it has to be manually configured and it gives me the port numbers. I add the numbers to the router and check the app again only to get the same error with a different set of public port numbers. Upnp has been turned off and back on but still no change. I’ve been on the phone for hours with Arris and they can’t figure it out. Tablo support says its the modem blocking the ports. I have run out of ideas so I’m hoping someone here may have an answer. Thanks.