I'm being charged for Apple Care of an iPhone I returned to Apple

Hi there

Question is in the title.

I had Apple care on my iPhone 11. Bought a new 13 from Apple and sent old phone in for part exchange. Foolishly assumed the apple care would transfer to new phone and carry on but have now been charged for the 2nd month in a row for Apple care on a device that Apple knows I don't own. ITried to find a way to remove the subscription online but don't see how to. I don't have the serial number or details any more.

Any ideas?

Thanks in advance.

X MJ

[Re-Titled by Moderator]