@Cedric1 

This feature is only currently available for Deco M4 with firmware V4. If you have M4(V4), this new firmware 1.6.0 will have to be installed manually. More about it here: https://community.tp-link.com/us/home/forum/topic/597476

  @Alexandre. My Deco's seem to be the v2 version.  Is the Source Selection ever going to be supported on these devices?  If not can you configure a M5 to connect to a M4 source?

Cedric1 wrote

  can you configure a M5 to connect to a M4 source?

  @Cedric1 

Unfortunately, you will need to have all M4 in your Deco mesh support this feature, to have it available. Sounds illogical, but it is the fact proven in testing. I had similar conversation with someone who had three M4(V2) and one M4(V4), while he can configure that M4(V4) to connect to specific Deco, he gets error message trying to save that configuration. Same will happen with M5 added to your M4(V2) Deco mesh, I assume.

There are no good options if firmware does not support that feature. Another option to those I already mentioned is to temporarily artificially weaken signal between Main Deco and remote Satellite. For example, you can start wrapping Main Deco in layers of household aluminum foil (make sure not to cover Deco vents) and at some point this should make Main Deco unreachable for remote Satellite Deco, which will then connect to Satellite with better signal. This connection will stay as long as Satellites and Main Deco are not restarted - so in case of brief power loss you'll have to repeat that again.