@ChadD 

The Deco units will switch back and forth. 

Have you done speed tests with your M5, when swapped to the x20 and done speed comparisons? If so, what speeds do you see?

What is your topology layout with and without the x20?

@Tony 

I used to have a 6 Deco M5s and I was experiencing intermittent problems using VOIP over a VPN and noticed using the web interface that the processor on the "Main Deco" M5 was always running high when I was seeing problems.  So I decided the new X20s may be a better fit to use since they have a better CPU and more memory along with the added features of 1024-QAM and whatnot.  So I swapped out 4 of my old M5s with new X20s (from the W3600 kits).  I made one of the new X20s the "Main Deco".  I also wanted to use these to try to increase speeds out to a detached granny flat.  

Anyhow, I'm having problems now where I didn't with the M5s.  At first each of the "signal sources" among each of the Decos in the Deco app was showing a strong 2.4g/5g connection with 3 bars.  Now almost all of them are showing "signal sources" of only 2.4g, and sometimes only 2 bars.  

Keep in mind my previous post where I used a signal strength test to figure out where to place the units and after placing them things were good for at least a day.  

Now, even if I walk right up to the main deco with the signal strength tester it won't switch to 5G.  It's very odd.  The M5s never behaved with switching down like this.  Previously with my signal tester I could walk right up to an M5 and it would show 100% strength with 5G at like -28db and then as I walked away it would go eventually to like -60db and eventually step down to 2.4g the further I went.  But then if I walked back to the M5 it would go back up and then switch to 5G.  It did that at first on the X20 when I installed it.  But now, whatever state it is in, it won't do that.  I wonder if something goes a little funny the longer the runtime is.  I imagine if I reboot them all it will work fine for a while.  I'll do that later in the day when my wife is done with work (I don't want to kill her connection).

If I run speed tests while connecting to the various Decos they do give me speeds generally in the 60-80Mbs range, which is 2.4G which makes sense when it says it is connecting at 2.4G.  I really think there is a firmware issue or something with these X20s.  If I plug in ethernet to the main deco I get speeds around 150Mbs, which is what I expect.

@Tony 

I ended up rebooting the "Main Deco" X20 that wouldn't connect to anything at 5g and it still didn't help.  I took it offline and replaced it with a different X20 and it has been working this last week.  It connects at 5g and only shifts to 2.4g if you walk too far away, and then it shifts back to 5g if you walk up to it.  So, the other X20 is definitely having problems.  It worked when I first got it going, but I can't say if it ever worked in 5g after the firmware update.

I tried "reseting" the bad X20 with a pin to the underside, and then I rejoined it when it was flashing blue, but it still wouldn't connect at 5g.  So, I think that unit is bad.

Over this last weekend I also had a wired gigabit ethernet backhaul wired to the granny flat for the Deco up there.  Turns out that new X20 up in the flat is having the SAME PROBLEM as my other defective X20.  It showed it was connected to Ethernet to the new "Main Deco" but connections to it only tested at 20-30Mbs and it was stuck in 2.4g mode to all connected clients, even my signal strength tester.  I had an old decomissioned M5 Deco laying about from the X20 upgrades and I swapped it in, and INSTANTLY all the devices had 5g connections and speedtested at 130Mbs.    

Something is wrong with either many of your X20 harware, or maybe this last firmware caused an issue.  To have TWO SEPARATE X20s with the same problem is more than coincidence.

@ChadD 

I can see the Deco placing the devices on 2.4G if there is no activity as what it should do is move the device over if the demand is high. So for example, if you were to download a large file the device would eventually be moved to 5G, and devices idle will be on 2.4G.

I would recommend following up with our support, and in the meantime, you could enable the guest network, and only enable the 5G to have a dedicated 5G network.