You have reached the maximum number of rules (12) that can be configured on a Zyxel T-56

@Jason van Odido You don’t need knowledge of MAC addresses and filters for this.
This is a bug in the firmware of the router.
The user interface promises 12 filter entries for both 2.4 and 5 GHz, but indicates that the list is full before this limit has been reached.
Only a new firmware version will help.

@EddyH0 I’m afraid that, if you want to resolve this problem quickly, you’ll have to get a different router yourself. Odido wants to test the firmware of their routers thoroughly before releasing it.
This thoroughness comes at the cost of speed: the b8 version firmware of the T-56 router introduced several new bugs, and was rolled out end november/beginning december. The cause of the problems was known by the end of december, and a new firmware was available halfway january.
Most of the customers haven’t yet received this new firmware, now, more than two months after a fix was available, and almost four months after the problems turned up.

If you need a fix quicker than that, my guess is that you’re on your own.
Maybe someone at Odido has more inside knowledge, and maybe the fix can be deployed quicker than with previous issues. But, going by Jason’s reaction, the problem itself isn’t generally known at Odido, so I wouldn’t hold my breath, if I were you.