@TWS-Columbus 

Hi, it would be better to put Ubee DDW36c to bridge mode and let Deco X55 work as the router.

Compared with disabling the Spectrum wifi, you will not have a double NAT issue.

Once Deco X55 works as the Wi-Fi router, you could set the same SSID/pw as my old wifi network.

3. Do I understand correctly that a 2.4GHz device will connect to the new network, even though it was originally a 5GHz network on the Spectrum wifi?

Do you want to know if a mobile device is capable of both 2.4ghz and 5ghz, which bands will it connect to Deco?

--If yes, since Deco uses the same SSID for both 2.4ghz and 5ghz, this mobile client is able to connect to either of them. The final decision on the preferred band would be made by the client itself.

4. Alternatively, I could also create another new network for use by my 2.4GHz devices using another legacy SSID/pw combo.

yes, it is ok. And for Deco, except for the main network, it also supports guest network, you could also enable a 2.4ghz only guest network if needed.

Thank you very much.

Best regards.