I would imagine calling would be best too. Although, if you managed to grab a deal back when the contract started, "in theory" it should continue at that price + the amount of increase out of contract, so if that's the case, in theory you can keep whatever you have now for £2, £2.50, £3 or whatever the change is out of contract more. No need to mess around with anything else if you want to keep the ipv4 address. Although, I don't believe anyone has really tested this, so you'll be the first.
But definitely call in to check and confirm. And also with regards to CGNAT, it's not a major issue for most games. Only a certain few using P2P connections will be hardest hit, and the only one I'm aware of is Mass Effect Andromeda and only the Multiplayer portion is impacted, and only when hosting the lobby. Joining others was fine. So I imagine it should be the case here for other games too. Unless if you are required to host, it should be fine for all usual content.