I'm running a single AC-PRO linked to Murfs Unifi controller (thanks very much). I have three Raspberry Pi 3 B+'s running Kodi connecting to an UnRaid SMB share for media sharing. I've been having problems with the Kodi clients buffering regularly when all three clients are running. I don't think it's the NAS - I've accessed 3 concurrent media streams from wired PCs with no problems. 

I'm assuming its a wireless bandwidth issue. The clients are all within 25m (two are only 15m) line of sight to the AP installed on the ceiling. I have noticed the clients connecting over 2.4Ghz even though they're 5Ghz capable. 

Is there anything I can configure from the Unifi controller to force 5Ghz (assuming the extra bandwidth should help)? To date I have enabled "Automatically Optimize Network and Wi-Fi performance" and I've seen one client connect to 5Ghz but still suffering from buffering. 

With two clients running they seem pretty stable.