@David-TP 

Hi!

Here is the setup:

I also have tried to use cell lock with carrier auto and elisa ->  won´t change band.

seems it "stuck" to B20 which seems to be most powerfull signal here.

  @David-TP 

Hi, N78 & N28 i can get around 500-700 Mbps before and now with B20 200-300Mbps.

I can only replicate this problem with firmwares 1.1.5 & 1.1.7

1.1.3 works with N78 and N28 

  @Will_K Did you find any solution...?

  I have similar problem. https://community.tp-link.com/en/home/forum/topic/706162?page=1

  @madvinegar 

We are still looking the case with engineer.

I looked your link and it seems you are having a samekind of thing. Is B20 very strong in your area ? i´m having -80dB when using it.

  @Will_K 

My best speeds are with N78 but the signal is stronger for N28 or N1.

I had a zte MC801A 5G router. I could choose N78 and it stayed connected to N78 all the time without changing to N1 or N28 so I had the best speed all day.

Now with Deco X50-5G I select N78 and after a while it changes to N1 or N28... What is the point to have the band selection option if the router does not keep it and connects to whatever band it prefers...?

I am very frustrated. I thought changing from the Zte to TpLink was the better option, but maybe I am mistaken.