An Apple Airport Extreme only will help with wifi issues. In my case the uniti is hard wired to the router, so I think another wifi access point will not help here.
I already did change the LAN port of the wall socket that leads into another LAN port into the router. But it is an idea to swap the LAN cable between uniti and the wall socket. But if it was an cable issue the problem should appear also with streaming from the local UPnP-server, shouldn't it?
Gert - up to you.
I only suggested a solution which works for other users.
And no - Apple Airprot is not only about wi-fi but about network management too.