Looks like you already have tried a number of troubleshooting steps, but see if any of the following can help:
Firstly, ensure that your iPhone is running the latest version of iOS. If not, please update it. Then, try to reset your AirTag. To make sure you don't miss any steps to do so, see the following reference article:
Next, bring your AirTag close to your iPhone and follow the on-screen instructions. If your AirTag still doesn't appear, try to restart your iPhone and repeat the process.
If neither of these steps resolve the issue, it might be a good idea to contact Apple Support. They can provide more in-depth troubleshooting and, if necessary, arrange for a repair or replacement.
To do so, simply select the "Support" link in the lower left-hand side of this forum's web page ... or go here: Get
Support