@eyeye 

It is possible that devices prefer WiFi6 from XE75 to WiFi5 from P7. It is also possible that what you have is just Deco app reporting issue: devices do connect to P7 but Deco app does not show that.

These two tests could help to figure which theory is right:

1. Hardwire something to P7 with Ethernet cable. If you don't see that device in Deco app connected to P7 over Ethernet, what you have is Deco app reporting issue. Obviously, device is indeed connected to P7.

2. Turn off 5GHz in Deco Main Network, leaving just 2.4GHz. From your screenshot, 6GHz is off - keep it that way. By doing that, you'll dumb down your Deco mesh to same WiFi capabilities for all Deco units. Then, place smartphone close to P7 (1 meter/3 feet away), turn smartphone WiFi off/on and see where it connects. If it does connect to P7, my theory is right about devices preferring WiFi6 over WiFi5.

Yes, all Deco can work together, but there is no guarantee their mix, especially from different generations, will work together the best possible way.

If you plan using XE75 6GHz - you should replace P7 with WiFi6e capable Deco unit. If you don't need 6GHz, then you can replace P7 with WiFi 6 capable Deco unit such as X50 or X55.