I can't reconnect the AirTag to my new iPhone
I can't reconnect the tag to my new iPhone, I just get the message "Could not complete AirTag setup" I have signed in with AppleID and I have reset the AirTag.
AirTag
You didn't mention it in your post, but I am assuming that you're using the same Apple ID with your new iPhone, as with your old one ... correct? I'm also assuming that you are not getting the "this AirTag belongs to another Apple ID" ... correct?
If so for both of these, then please double-check the section, titled: If you can't set up your AirTag, in the support article you arrived here from. Here is a reference to it again: Set up your AirTag using your iPhone, iPad, or iPod touch - Apple Support