I have been using OpenVPN Server on a Raspberry Pi 4 for yonks, it was fine with Virgin and then also fine with BT, still is today, there was quite the difference between the two also, Virgin using DHCP to give me an external IP where BT PPPOE. When moving from Virgin I had to update my DNS to dynamic as BT doesnt use a static IP's for residential, as my VPN clients devices only look to connect to the DNS name they dont really care whats behind that, so long as that DNS record points to my router and then my router will forward the UDP port to the local IP of my Pi VPN Server.
With BT now my router will automatically update my public A record to resolve to whatever external IP is assigned to my PPPOE interface, so the client config remains the same.
Perhaps Plusnet had a static IP, maybe you are using dynamic DNS already I dont know but you say they connect but cant browse the internet, I assume LAN also. Maybe firewall rule needs updating, given that the VPN is configured directly on the router there maybe an automatic firewall rule that was created. How is your DNS setup on the VPN server also, I guess automatic? Have you tried just deleting the config and re-building it creating new OVPN files? Just wondering also if the default route might be causing and issue which will be different after moving.
Someone else mentioned this also but I did have an issue once when the location I was at was sharing the same subnet as me. It doesnt matter what private address your client gets for its VPN connection if the parent interface is on 192.168.0.0/24 where ever you are and thats the same subnet you have at home it wont work, well internet should work but accessing local clients wont because when it looks up anything local it will look up on the network where you are at and not over the VPN connection. For this reason I changed my home subnet to 192.168.84.0/24. But you said you cant use the internet and this theory would only apply to trying to get at your local clients from a remote connection on the same subnet. It doesnt matter if the VPN NIC has a 10 address, the parent interface will be the one that matters. The internet wouldnt work however if perhaps you are running your own DNS server at home and you were somewhere else sharing the same subnet.
Bit of a brain fart I know but trying to think about what could be the problem.
Edit: Looks like you have confirmed that the VPN Client is not reaching the VPN Server as I was writing this, in that case I would be first looking at your external IP and if your clients are expecting a static IP which is still your Plusnet one, you'll need to update that to the BT IP on the client site and i'd recommend utilising dynamic DNS then so if your IP every changes your router will automatically update your A record, therefore your clients can use the same DNS address everytime. Theres a ton of options to consider for that, too many to post now!
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