Why is AirPlay/Print not supported?

Perhaps I have overlooked it, but I cannot find a way to activate an mDNS reflector in USG Flex 50H.

It looks like a very common usecase to have trusted devices in one VLAN and unsecure devices like printers, TVs, … in another VLAN.

Could it be that ZyXEL does not support this option - and if not, why?

Or it there any trick how I can activate it?

I would appreciate some help.