Emigre wrote

I hacked out a solution today by adding a old tp-link range extender to my Deco Mesh network and constraining range extender's wifi broadcast to 2.4ghz. In effect I've had to create an entirely new network exclusively for the aircon.

@Emigre 

You can't do the same with Deco Guest network: enable only 2.4GHz and make aircon connect to it? Just curious.

@Alexandre. 

The problem is that the Tplink M5 deco have in Europe at least the ability to work 2.4ghz Wi-Fi on channels ranging from 1 to 14 and there are devices, in the case that we are talking about air conditioning, that their Wi-Fi works only on the channels 1 to 11 due to its influence from the North American and other countries. Therefore, the WIFI of one and the other cannot be synchronized. Tp.link had to give the option to choose the Wi-Fi channel in its deco and at least the particular option, to cancel channels 12, 13, and 14 of the 2.4ghz frequency

@Alexandre. I tried that and I couldn't get the aircon to pick it up.