By default, Untangle supports SNMP on non-WAN interfaces. I want to use SNMP from a WAN interface. I assumed that it would suffice to add an Access Rule and did so with Destination Port 161, Protocol UDP, Source Interface Any WAN. My SNMP driven monitoring tool (LibreNMS) failed to see the device when I pointed it at the external IP address. I also added a Filter Rule to permit Destination Port 161, Protocol UDP. Still no luck.
I've read old forum entries on this topic but don't know if anything has changed with more recent Untangle releases. I am running version Build: 14.1.1
As an aside, if I run an OpenVPN session to the device with the OpenVPN client being the machine that hosts LibreNMS I am able to get SNMP to work on the LAN side of things, so I know that SNMP is functioning, but not on the interface I seek.