I discovered one thing, 5Ghz actually works but it’s the problem elsewhere. I state that this S912 is connected to a repeater as described in the post:
In this repeater I have activated the MAC filter so that only the MAC addresses in the list are allowed to access it. However the 912 MAC was already in the MAC whitelist so it had to access it. I discovered this by temporarily disabling the MAC filter on the repater. I added the 912 MAC again by overwriting the existing line and saving the changes. Having done this, the 912 connected to the repeater with the MAC filter active. So I suspect something that got stuck in the repeater.