Can you backup using your mobile data?

5G capable iphones 12 and 13 on iOS 15 can use mobile data to back up to iCloud, even if it is only 4G

Unfortunately at the moment it only works manually (Backup now) not automatically.

i need to back up my iPhone 7with mobile data on iOS 14

You can’t. You need an iPhone 12 or 13 running iOS 15.