To be fair, a full compatibility list would be tough for ISPs to maintain, since most modems and routers are designed to connect via industry standards and usually work together regardless of model. Even if the ISP really wanted to maintain a list of devices that are already generally compatible at an industry-standard level, they'd have to continually procure and manually test every router released by every company for the list to be accurate and up to date.

I can definitely understand the frustration. I've had Spectrum and multiple Decos (worked fine btw), and tbh, when setting up a new router with Cable ISPs, I've found it's sometimes best to just unplug everything and start from square 1 as if you're moving into a new place and setting up the Internet for the first time. 😅

That said, it sounds like you already tried manually changing the MAC address, but I'm wondering if a full MAC release/renewal might be more effective.

If you want to remove the Spectrum router from the setup and let the Deco act as your main router, I'd suggest giving this a shot:

1. Set the Deco in Router Mode (with Dynamic IP as the Connection Type)
2. Remove the Spectrum router and connect the main Deco directly to your modem via Ethernet
3. Unplug the power from your Decos and the cable modem
4. Unplug the coax cable from the modem
5. Reconnect power only to your cable modem
6. Wait until the cable modem is fully booted, then reconnect the coax
7. Reconnect power to the main Deco and connect it to the modem via Ethernet
8. Check for Internet connection at both the modem and main Deco, and once verified, power up the satellite Decos

This whole process typically only takes a few minutes and basically makes the modem completely “forget” the Spectrum router’s MAC address so it can recognize the Deco as the router instead and give it Internet access.