iCloud Backup Failed

I need to transfer my data to a new iPhone and I haven’t done a backup on my old phone since 2018 because I didn’t have enough space in my iCloud.

I have purchased additional space for my iCloud and went to create a backup. It spent ages whirring then said could not complete the back up, it then refused to make any progress over and over so I turned off iCloud backup hoping to turn it back on and fix the issue. Now it won’t even turn back on and just says ‘iCloud backup failed there was a problem enabling iCloud backup.’

I really need to resolve this issue can someone tell me how to turn it back on and create a successful backup once it’s back?

Updating my phone and logging out of iCloud are not options (I don’t have space for an update hence the new phone, and I don’t want to lose my iCloud notes)