There's an easier way. Just open Syslog Viewer on the Solarwinds server and see if your syslog messages are coming in. If they are then all is good. If not, then the device is not sending them correctly or it's getting blocked upstream.
The OP had the syslog messages coming in but as the IP addresses differed, the information was not populated within NPM. Source IP of the syslog message needs to be the same as the monitored device's IP in NPM otherwise they don't match up.