I have a Ubiquiti USG (soon to be put to pasture) that has multiple VLAN's on it already. The default VLAN is the subnet as shown, yes (*.255/24). I have already verified I can ping from, say, my laptop on one of the other VLAN's in the network to devices in this *.255 subnet, so there should be no firewall issues at play here.
The port is connecting to another switch (which, again, is a Ubiquiti switch soon to be retired). That port is configured with a profile that sets "All" VLAN's to be tagged, or so it seems to me. I spent hours on this last night trying to determine whether that's indeed the case, and it seems to be. But I'm not sure how I can inspect frames, etc. to be able to debug it further.