Thanks Bob. It's a co-op mod for GTAV for PC -https://www.gta5-mods.com/scripts/multiplayer-co-op-mod#description_tab - apparantly it uses UDP 4499 (as mentioned in my post 'Under 'Manage games and applications' I've tried port 4499 as 'Any' as well as individual TCP and UDP entries'). Would you reckon that the next step is simply to try getting a distant PC to access?
It's all a balance. In my case there were three people at once trying to access the router wirelessly and in addition 4 fixed items of equipment in the house that need reliable remote access, the plusnet router simply can't hack it.
As noted upthread it is a mystery why Plusnet claim the router has the capability if it is not really there, and equally why they can't get Sagem who are the actual manufacturer to sort it out given that they put so much business their way.