Got the same problem, Plusnet quote data rates that are impossible for the local loop, faults on the broadband seem to spill over onto the 5Gig WiFi circuit, the only way I could get my Sky Mini TV units to work was to get 4 TP-Link power modem units.
The Plusnet supplied router appears to be a BT 5a, one easy way to improve the WiFi is to get a BT 6 unit try eBay.
The faults are known to Plusnet, if you have a near perfect local loop you may not see any problems, if you are lucky.
The perfect fix is FTTP (Fibre To The Property) which Plusnet can't supply at the moment I believe
