as far as I can see, Voxel firmware (and probably stock also) does have support for VLANs.
(advanced settings -> VLAN/Bridge settings)
WARNING, not sure if it is related, but to write this post yesterday, I clicked around a bit in that section, without actually saving anything.
But shortly after that, I could no longer connect from WiFi to wired devices.
Then I rebooted the router -> after that my internet connection would no longer work. (didn't get an IP anymore, and ifconfig of ethwan (or brwan, don't remember) showed only TX and no RX traffic.
After a lot of troubleshooting (trying different cables, connecting modem directly to a laptop, etc,etc), I noticed that the checkbox "Enable VLAN/Bridge group" was still enabled.. -> apparently this checkbox already becomes active without pressing Apply ??
Anyways, disabling that checkbox again got my internet back.
But the connecting from wifi to wired still would stop working after some minutes / hours.
Then I started messing again in the VLAN settings. And after that, I could no longer reach the router.
So ended up doing a factory reset...
(and then still spend a lot of time fixing stuff, because the wireguard config file that I tried last week, was still present on my router, so Voxel firmware did try to start wireguard, but this would fail because my Wireguard Server in Azure was powered down.
But somehow wireguard start script removes the default route even when connection is failing to establish a connection.
so even after factory default, I could not yet access internet.)
TLDR: Do not mess around in the VLAN/Bridge settings unless you know what you are doing, and might need a factory reset to recover. (so make sure you also backupped all important things like reservations and port-forwardings)