My friend switched to Plusnet 3 days ago from BT. I setup his router for port forwarding for his CCTV setup exactly same as with the BT router.
He now has a very strange issue that he cannot view his CCTV remotely while on 4G with o2, I tested with my Three mobile and also a Vodafone one and this works absolutely fine. We then tried another mobile that was on o2 and the same issue occurred.
I can't answer your question but would suggest that you reinstall the BT Hub and see if that works. They are much better devices anyway and may of us use a Smathub 6. You will find setting instructions for connecting to Plusnet pinned ar the top of this Router board.