Hi @Web ,
Just so you know, we just found your thread and can chime in on exactly the same issue. See our TRB500 - 5G NSA not possible after upgrade, 5G RSSI N/A, Type jumping between 5G and 4G+ Thread.
Exactly same behaviour as yours on the newer firmwares. We had 5G running for the last 3 years on the device with NSA successfully. Now after updating our device we have the same problems as you. For now maybe try downgrading, we are looking for the correct firmware which works for us (right now 07.19.4 looks stable on TRB500)
Also the upgrade Tool is completely broken and updating to the wrong version. Download the file manually for your device and update from file.