I take it you want to access the server using frequencysniffer.ddns.net
Domain names are tied to your network outside address and not the inside address so you will have to setup the router Port Forwarding to allow access from outside your network? Use
Open Port Check Tool -- Verify Port Forwarding on Your Router to test that the Port Forwarding is setup correctly. The ProScan manual should explain the setup. If not, see this manual on the port forwarding setup.
Uniden BCD536HP Wi-Fi Stream and Remote Access Via Proscan Web Server Ignore the BCD536HP part and concentrate on the Web Server part.
If all is setup correctly then enter in frequencysniffer.ddns.net:5000 in the web browser. Assuming the Web Server is using port 5000.
If using a cell phone browser then turn off the cellphone WiFi so it will use the outside address.
Keep the Publish Server option unchecked.