RAX 75 : Problems with 5 GHz - two issues

I have an RAX 75, set up to use Smart Connect.

1. Devices are constantly flipping back and forth between 2.4 and 5 GHz. Most of these devices are completely stationary (TVs, music streamer, etc.), and relatively close to the router (the router is almost directly over some of the devices, on the second floor).

I've tried using various channels, including DFS. Using DFS channels, a couple of devices switched from 2.4 to 5 (but reverted back to 2.4), probably due to the higher power output.

The only reliable 5 GHz connection is a Samsung S-10. Every other device flops back and forth (and the S-10 moves around more than any other device, including outside the house...)

I guess I can try turning off Smart Connect and using two different SSIDs, but it would be nice if the feature worked. Is there anything I can try?

2. I'm still trying to pin this one down, but it appears that sometimes devices on 5 GHz can't see devices on 2.4 GHz. For example, one laptop was able to see a music streamer (SSH, ping, etc. were OK) while both were on 2.4. The laptop switched to 5 and completely lost communication with the streamer. During this time, I tried from two other laptops on 2.4, and it communicated with the streamer with no problems.

Sometimes, however, communication from 5 to 2.4 works.

Guest Network isn't enabled, but I did enable the option to allow guests to see each other. It shouldn't matter, but I read somewhere on this board that it might help (it didn't).