Separating or Disabling 5Ghz and 2.4Ghz on Mesh WiFi 6 System

Nighthawk Mesh WiFi 6 System AX1800  (model number refused to type in, above)

My problem feels simple, though the solution seems elusive.

I have a houseful of  Amazon Echo devices (along with several "smart" switches, a Ring doorbell, a MyQ garage door). I like to play music on ALL of the speakers at the same time, and was able to on my older router. I bought this new router in order to reach the garage door more effectively. After weeks of frustration, I got the garage door hooked in, using the guest network (nothing else would work).

But my Echos will not all join the "everywhere" group, and the problem seems to be the 5ghz vs. 2.4ghz conflict. A wireless printer had the same issue, though I resolved that in a way that eludes  me. Don't care, it works now.

But is there a way to either TURN OFF 5ghz, or split it so that the Echos don't try to join different bands? My understanding was that devices pick the thing that they work with automatically. This has not been my experience.

Thanks in advance for any help. It's driving me up the wall. --Jamie in Reynoldsburg.