I needed to replace my previous M5 "Main Deco" with something more powerful as I thought it could have been the source of intermittent VPN issues. I purchased the W3600 pack (which I think is the same as X20's) believing they have a better CPU and more RAM. Anyhow, they have made matters worse.
The new W3600s (X20s) continually drop down to 2.4Ghz. All the connecting Deco's are almost all now connecting to each other over 2.4Ghz as well. I used a cell strength app on my phone that I've used plenty of times in the past, and now even if I go right up to the "main deco" it won't connect at 5Ghz. It's almost as if the W3600s all shift down to 2.4Ghz no matter what.
Also, I have an M5 in my entry way that used to be solidly connected to the "main deco" M5 at the full 5Ghz (full bars in the Deco app) all the time since it's only 20ft away from the main deco with clear line of sight, but now it also only connects at 2.4Ghz to the new W3600 main deco.
I have made sure that 5Ghz is enabled (I never changed it), and it seems that if I reboot they will connect at 5Ghz, but then over a couple days they revert to 2.4Ghz.
There is something very wrong with the W3600s (X20s) at this time. I hope it's only a firmware problem. Please fix it. I never had this stepping down issue with the M5s which were in the same locations. I tested the signal strength using a cell phone app when I replaced the M5s with the W3600s (X20s) and they were at full 5Ghz strength when I replaced them as well. I assure you there is no reason besides firmware or bad hardware that this is happening.