Setting up UTM 9.1 in a inter-routed VLAN environment

Hello,

I'm having trouble with setting up Sophos UTM 9.1 in my lab environment.

Basic network topology is:

VLAN1---------
....................|.........(VLAN4)
VLAN2----L3 Switch----eth1-UTM-eth0----Internet
....................|.......................|eth2 (for management)
VLAN3----------------------------

All VLANs are inter-routed and can ping the UTM interface on VLAN4/eth1.

The UTM can ping internet addresses, and so can devices on VLAN4. This has lead me to believe the UTM is blocking connections except those on the "internal" interface.

Does anyone know how to add multiple VLANs to the "internal" interface?

Thanks in advance :)