5th is controlled electrically, with a hint of 1980s electronics. It is controlled by the HGS box.

You need to check any error codes returned by the HGS. And, of course, that the HGS is properly plugged. I lost 5th when I installed a used eBay HGS and didn't plug it in properly. After that, I cleaned contacts, cleared error codes and 5th is back.

And so is 4th because that can be an issue.

Generally, problems with 4th/5th are related to the electronics, not the 722.5 gearbox itself.

Also, ensure that the 4th/5th switch is working - in my case, I replaced it for about 20€, built a led blink code device, cleared the code and 4th returned as it should.

Good hunting.

RayH