Service Will Not Activate & Devices Won’t Connect

Hello,

I purchased an Xfinity internet package last week, and the self-in stallion arrived yesterday. I am using my own modem and router (Arris SB6183 and a Netgear Nihhthawk). The modem and router and working fine, and I am able to connect my devices to that WiFi signal. However, for the life of me I CANNOT get Xfinity to activate my devices. I am using a cellular signal to get to the activation portal. I can’t plug my laptop in to my router with an Ethernet cable, but I didn’t think this was necessary. Any help? Customer service has been MIA.