Hmm.. Sounds like your phone is actually turning off the data service when you tether to it? Check your settings to make sure this isn't the case, as this may be a default behaviour to stop you using up your monthly allocation! I'm almost android exclusive and avoid anything apple, but i've observed this behaviour with many mobile devices, regardless of manufacturer! You may also possibly have a poor data signal where you're currently trying to use your phone. If you're in a house, you could always try upstairs to see if that improves reception?
If you require further help let us know, although i'm unable to offer any remote help!
This link may be of use to you https://devices.vodafone.com.au/apple/iphone-12-pro-ios-14-1/connectivity/use-your-phone-as-a-person...
Kind regards,
Lee
