There are two changes you need to make in your configuration for this public interface to allow SNMP from your Solarwinds sever.
1-community - Add Solarwinds NAT ip address
Configuration > Report Settings > SNMP
Create or edit your community name aaa
2-allow snmp on interface connected to the internet that you are polling
Network > Interfaces > List
edit the interface that the SNMP request will come in on
Check that SNMP is allowed on this interface