Unfortunately, this is as far as my knowledge of developer iOS betas goes, so I can't personally assist you any further :-( Best wishes! -NKTMAT.

Just to make sure, ensure you are signed in on the device WITH the account that you have registered as an Apple Developer account. I have seen this before, and the issue turned out to be that the person wasn't signed into the correct iCloud account associated with their developer Apple account. Sorry that you've been trying to fix this for a while, this is the best I can do at the moment, as I'm not an Apple Staff team member, nor have I seen this issue as a common thing. Good day! -NKTMAT!