Question about Galaxus 10 Gbit/s Fiber

The Zyxel is in the basement and I have several Ubiquity APs distributed throughout the house. To set up a guest Wi-Fi, I need VLANs on the LAN side of the router (which the router does support). However, this has been disabled by Galaxus, apparently for technical reasons. It is not entirely clear to me what the technical reasons are for restricting the router so severely on the LAN side. Why are useful features deliberately disabled? I would like to avoid double-NAT and not use an additional router. Is there a sensible solution here that I am overlooking?