@David-TP 

TP-link Onemesh can do separate mesh SSID for 2.4 GHz and a separate mesh for 5GHz. 

Maybe TP-Link Deco has the same options in settings?

When I had my Wifi 2.4ghz and 5ghz as the same SSID my Samsung phone would connect to 2.4 Ghz band in the driveway and never let go, sometimes it would use both 2.4ghz+5Ghz bands together but it would never get onto the 5Ghz band alone, which was a problem as I would get massive interference from the microwave oven on 2.4GHz when streaming the news tv in the kitchen. It seems even if your phone is using 2.4Ghz+5Ghz bands together, inference on the 2.4Ghz screws everything up and streaming video would not work. I needed the ability to be on 5Ghz alone.

So I only use the 2.4GHz mesh for old devices that have no 5GHz. I use the 5GHz mesh for everything else, which gets no interference from the microwave oven in the kitchen.

The key thing is making sure "Smart Connect" is turned off. Basically what "Smart Connect" does is join your 5GHZ and 2.4GHZ together. Whereas for me I wanted my 5GHz mesh separate.

I have a TP-link AX20 router and a RE505X extender.

The way to do it on TP-link Onemesh is set up your router with two different wifi SSIDs for 2.4 and 5, i.e. "Wifi-2.4GHZ" and "Wifi-5GHZ". Then make sure in wifi settings "Smart connect" is off as this will keep your 2.4 and 5 separate. 

Make sure the router onemesh setting is turned on. Then factory reset your extender. Then start up your TP-link extender, in the setting it will ask you what 2.4ghz network to extend, make sure the 2.4ghz network you select has blue writing saying "Onemesh", put in the password for this. Then it do the same for the 5Ghz network again it should have blue writing next to the SSID saying "Onemesh". If it does not have this "Onemesh" writing next to the SSID then you need to go back into router settings and make sure "Onemesh" is toggled "on".

Then it should be all set up. I would go into router wifi settings again and make sure that "Smart Connect" is turned off and hasn't turned itself back on in the Onemesh setup. ODFMA is also turned off for me.

Basically what happens is my smartphone connects to "Wifi-5G mesh" and when I walk through the house it will hand me off to the extender "Wifi-5Ghz" mesh all in the same mesh SSID when the signal is best from the extender and I don't have to click anything to move onto the extender. And it will not turf me onto the "Wifi-2.4GHZ" mesh which has a separate mesh SSID and I make sure on my phone that my phone is set to only "automatically connect" to "Wifi-5GHZ" and NEVER automatically connect to "Wifi-2.4GHZ".

On the odd occasion if I am far in the backyard the 5Ghz will not reach, and in that case on my phone I manually switch over to the "Wifi-2.4GHZ" mesh. I rarely need wifi in the backyard, so I am happy manually doing this. But in the house I am happy using 5GHz only.

  @Avanti the options is there in my XE75, but under a separated menu.
 

I also needed to separate 2.4Ghz and 5Ghz networking for my IoT stuff, but I found the workaround in the IoT menu where I could create a dedicated 2.4Ghz network and then under the normal WiFi menu I just said it should be a dedicated 5Ghz network. Know I have 2 separate networks that can talk together etc. so far I see no speed limits or separation on the IoT network, so it must be working as expected.

Bought the X10 2-pack yesterday and returning when I next visit the shop. Unacceptable not to split 2.4 and 5 IMO. Just configuring either doesn't work for my set up at home. Back to my ASUS ZenWifi Mini which, despite giving me the occasional headache with buffering (the reason I'm switching), are far better in terms of functionality, TP-Link are a disgrace for releasing what I consider to be a crippled router / AP... 

What the wireless manufacturers don't understand is that while an IoT device will steer to the 2.4Ghz network your smartphone (iPhone) won't and the IoT device software expects that you are on the 2.4 GHz network so you can't configure your IoT devices.