Cancelling my internet has resulted in mobile contract being more expensive - is this correct?

We signed up on the 27th of November 2024 for a 24 month contract (2 phone numbers).

Part of the deal was that we get 15 euros off per month for the duration of the contract.

We moved into a new house in March 2025 and while we waited for glass fibre internet to be installed (not provided by O2 in my area) I took out a 1 month rolling internet contract with O2. 

We have now finally had the fibre cable installed so we spoke to O2 to cancel our internet, they have cancelled it but they have said that the 15 euros per month deal we have with our mobile phone is no longer valid and will be going up to the full price. 

This isn’t correct surely? They are 2 separate contracts. Is there anything that can be done? That’s 360 euros over the course of the next 12 months until contract expires.