@FLPete,

Even if you have both bands enabled, there will still be only one network displayed that you will connect to; however, since you're other 2.4 Ghz devices cannot connect, I do not think that this is happening. 

Whenever someone upgrades their modem, I always ask: Is the network name the same as the previous?

If it is, I would recommend resetting the devices that will not connect and see if they now can be configured to the network.

Lastly, and most importantly, if the modem you installed was a modem/router combination, please ensure that you have disabled the wireless functionalities and placed the device into Bridge Mode, as having two networks operating simultaneously can cause severe issues.

Another Option, would be to try and switch the deco units so that one of the satellite units is the main unit. 

To get some more information:

What is your Modem and Service Plan from Spectrum?

Can your 2.4 GHz devices connect when the satellite nodes or disconnected from the network and the main unit is the only node providing a wireless connection?