Hi @Charlotteh1 and welcome to the EE community
Firstly, by means of general advice, EE mobile contracts don't arbitrarily end on a fixed date, rather they are rolling contracts with minimum terms. Until & unless you have explicitly given 30days notice to disconnect a particiular number, the contract will roll on.
Secondly, changing your phone while keeping your number is known as an upgrade. In order to do this, you need to have bought your phone as an upgrade, and it sounds like what you've done is bought a new phone with a new number. EE will not officially port numbers between EE contracts, so you have 2 choices on how to proceed:
1: Assuming you bought your new phone online or by telesales, you need to use your 14day statutory "change of mind" return, to cancel that new contract, and then purchase your chosen phone again as an upgrade. Whether you buy it from EE direct or a third party such as Fonehouse is immaterial - what matters is purchasing it as an upgrade.
2: You could use the PAC process to move your existing line to another network for a short period, and then use the same process in reverse, to move it back to pay-monthly replacing your "new" line. You can do this using either another network provider, or also EE PAYG.
