Untagged LAN is not available in VLAN Trunk mode in the following models:
Balance 20
Balance 30/30 LTE
Balance 50/50 LTE
MAX BR1 HW1
MAX BR1 Pro HW1
MAX BR1 IP55 HW1
MAX BR2 IP55 HW1
MAX BR2 HW1
Surf SOHO HW1
Custom VLAN is not available in VLAN Trunk mode in the Balance 305, 380 or above, MediaFast 500/750