I'll be honest that I had a time setting this up and don't remember a thing on how I did it but it was working for months and suddenly stopped working in December with no changes to any products other than a software update to PRTG. I tested SNMP with PRTG's tester and locally it works, remotely from the other side of the VPN it doesn't get any response from the firewall. Going to bounce the firewall tonight but can you think of anything else that would just make it stop communicating? Certainly wouldn't seem to the the PRTG update since the tester fails too.
Any references to getting SNMP over VPN working would be helpful as I'm sure I can't find my notes. Endpoints are Netvantas.