@Golfetta 

Different M4 hardware versions is the problem. TP-Link only has M4 V4 firmware that allows to configure how Deco connect to each other. When you added M4 V2 to your mesh, not only they do not have an option how to connect, but they have disabled that option for M4 V4. When trying to configure M4 V4 you will see an error message saying "Unable to save configuration."

If you need to configure how Deco units connect to each other, here is the way:

1. Delete new Deco M4 V2 from your Deco mesh. See https://community.tp-link.com/en/home/kb/detail/330 and follow "Reset a Deco unit"

2. With only Deco M4 V4 left in your Deco mesh, verify that you can configure and successfully save how Satellite Deco connects to other Deco. Meaning: that feature (signal source selection) is back and working properly. If not, escalate to TP-Link tech support as direct email or a new forum topic.

3. Return new set of M4 V2 to Amazon and buy a set of M5 instead. Deco M5s have signal source selection for all firmware versions. In performance and specs they are similar to M4.

For the reference, here is the current list of Deco models and hardware versions supporting signal source selection. You must have Deco mesh consisting of these models and their hardware versions only to have that feature work for you: https://community.tp-link.com/us/home/forum/topic/597476