@Handpol 

By default the Deco uses channels 1-11 as 12 and 13 are not supported in all countries.  Its likely your deco is on channels 1-11

You could install WiFi Analyser for Android and this will tell you what channel the deco is broadcasting on.

If its not connecting, this may actually be the encryption type.   WiFi6 devices like the Deco x20 use WPA3 SAE, this is not always compatible with older devices, try changing the encryption to WPA2 AES and see if that fixes your issue

  @Philbert 

Thanks for the reply, I had already used wifi analyzer and de 2.4ghz band is allocated to channel 12. My JURA adapter is only accepting a range until 11, like many adapters. Unfortunately I know it is not possible to change channels manually and can only hope that the current channel will become busy, in order that the network analyzer changes it automatically. Not a big change, as I have a detached house. Changing encryption does also not help btw. 

If anybody knows a work around to push/force the DECO to another channel, I appreciate a reply.

Until then, reasonably happy with the stability of the DECO's, but not able to change channels sucks, like many times pointed out in this forum.

  @Handpol 

The workaround is to find (borrow) WiFi router that allows channel change. Configure it to broadcast on channel 12. Place it near the Main Deco. Run Network Optimization on Main Deco - that should force Main Deco move to a different channel.

  @Alexandre. 

You may find that an Android phone would also do this, some have hot spots that are configurable for different channels