I suspect you are seeing one of two problems (or possibly both):
SNMP is not listening on the public interface of your router.
SNMP packets are being blocked outbound from Amazon OR inbound from your ISP. I wouldn't be surprised if this were the case.
I'm not a VPN expert, but setting up a VPN might work. You'd need the VPN server within your network and the VPN client on the ec2 instance. That said, I don't think that's wise from a security standpoint; if your ec2 instance becomes compromised, your whole network is compromised :(
For network monitoring, you really want the Observium appliance on the same network as is being monitored so I'd run a VM or a low cost server within your network.
I'm no VPN expert
I suspect you are seeing one of two problems (or possibly both):
I'm not a VPN expert, but setting up a VPN might work. You'd need the VPN server within your network and the VPN client on the ec2 instance. That said, I don't think that's wise from a security standpoint; if your ec2 instance becomes compromised, your whole network is compromised :(
For network monitoring, you really want the Observium appliance on the same network as is being monitored so I'd run a VM or a low cost server within your network.