Apple TV showing old AppleID when I log in. Can’t rent or purchase anything.

I changed my AppleID from gmail to ProtonMail about a year ago. Today I installed the AppleTV on my streaming stick, and logged in with my iPhone (which is using the new AppleID), but it came up ‘Logged in with AppleID ********@gmail.com’.

This email address is no longer associated with my account in any way. Not even as a secondary email or alias. In fact I deleted my whole gmail account.

Because of this glitch I am unable to rent or buy films via the AppleTV app.

How can I fix this?