@Randomcdn 

If you have Ethernet port in that room, which is connected to the rest of your home LAN, there is another option.

Get the most inexpensive 2.4GHz WiFi router you could find. In my region that would be TP-Link N300 WiFi Router (TL-WR841N).

Configure it just like regular router. Make sure it has different SSID from your Deco mesh. This is obvious but often not understood part: this router SSID must not be the same as any of SSIDs from Deco mesh WiFi networks.

Connect router's WAN port with Ethernet cable to in room Ethernet port. Configure smart thermostat to connect to router SSID. You now have 2.4 GHz WiFi network just for that one smart thermostat.

  @Alexandre. 

Thanks for this. I'm in Canada and that router is which is cheaper then the wifi extender I was looking at

  @Randomcdn 

Me too, and I checked its price on Amazon Canada yesterday.

Usually smart thermostat apps work with thermostat through the cloud, meaning just configuring this router as router should be enough. In case app requires direct access to thermostat through your network, you may need to configure that router in AP mode. I checked its specs, it supports AP mode. This is a bit of extra work, but if required you do it once and will forget about it.