For reference: 2021 mustang 4 inch to 8 inch sync screen - SYNC 3 Support / Ford Update Support - CyanLabs Official Community
From what I understand you changed the APIM and screen in the Mustang to the 8" version and loaded the appropriate software. This would have broken the VIN and APIM/Modem links for FordPass. To get the systems working again you will need to reset each component in the system.
Go to your FordPass app and remove the vehicle from your account. Close the FordPass app and reboot your phone. (This is to clear the app cache.)
It may take up to a day for this request to process thru the FordPass system and delete the vehicle from your account, so this may require patience. You can check this at the Ford Owner site under your account.
Go to your vehicle with FORScan and perform 2 functions:
First, perform a Master Reset on the Sync system, See doc below.
Second, use FORScan to reset the TCU. This will be in the Tools menu.
Once this is done shut down the car.
Now follow the instructions in this document…You don’t need to do the steps above again.
Reauthorization of FordPass Connect TCU.pdf (527.5 KB)
If this fails, you may ned to perform the following…
FordPass Connect TCU Authorization Thru FORScan TCU-J_K_L.pdf (789.7 KB)
Remember that the vehicle must be running for the modem to be activated.