Just went into one of the Max Transit 5G I have here at the shop. Went to Networks>New LAN then entered IP address 192.168.100.1 255.255.255.0/24
Gave it a Name and VLAN ID (100) DHCP Server Enabled… Set start IP 192.168.100.20 - 192.168.100.254… Assign DNS Server automatically and save. Created VLAN Only ID 100 in UNIFI, Went to a UBNT switch, tagged Port 14 with VLAN ID 100 and then plugged my laptop in… my laptop was given IP 192.168.100.20 from DHCP…
Works exactly as I outlined above… If its not working for you, then you have something misconfigured unfortunately