@nickarsow 

If you have two different 2.4GHz WiFi networks (which are in your case Main and Guest) and device can connect to one but not to another, it usually points at the following that is different between them:

1. WiFi password;

2. WiFi security settings (WPA, WPA2, etc).

For the purpose of troubleshooting set exactly the same password on Main Network that you have on Guest network, set same Security on Main that you have on Guest, double check again 5GHz is disabled on Main Network and try again.

  @Alexandre. Thanks Alexandre for you quick reply.

And yes, both passwords and security settings are the same. The problem is that Tuya sensors can pair to guests network, but not to the Main network.

I'm pairing the sensors via my mobile where beforehand I switch to Guests net, then pair the sensor without problems. If the mobile is connected to the main net, pairing is impossible.

I also tried to disable the fast roaming, but with no success. Also double checked that 5G is disabled and only 2.4G is enabled.

  @nickarsow 

Actually, one more thing: anything special or different about SSIDs for Main and Guest? Non-English symbols for Main but not for Guest? How about swapping Main and Guest SSID names in Deco app?

If that is not it, my apologies I couldn't assist you.  It's above my competence. What you are saying is you have two identical WiFi networks, and device can connect to one but can't to another.

  @Alexandre. The name of the guests net was given by the system when activated. I first tried to pair the sensors via the main net, and later I activated the guests net and then tried via it and succeeded. Now sensors are visible via both nets. The problem is just with pairing.

  @David-TP Hi David,

Video and explanations sent to your e-mail.

BR

Nick

 Hello, I have the same problem or maybe a little bit different. Till yesterday I had my TUYA sensors connected to the other router. Yesterday I changed the router and installed deco M4 three devices. From yestarday my tuyas do not work. I have tried with 2,4GHz only with 2,4GHz and 5 GHz nothing. I noticed that the TUYAs are acctive only when I am very close to them. I mean I have 10 TUYAS in windows to contral curtains. When I get close to the tuya it is working, when I go to another the last stops working and the close one is working.

I have M4 as AP, M4 conected LAN via switch to LTE router with static IP. In the router wifi is switched off. Can You please suggest sth bc my wife is getting crazy :)

Thank You for Your help. Hubert