Seeking developer to solve unicast UDP NAT traversal

I hope I'm using the correct terminology here, but we need a developer to get unicast UDP packets to traverse from the WAN interface to the LAN interface to a specific NAT'd IP address and port. The traffic need only work one way (WAN to LAN).

There is potential for some future development work as well, but this is the most timely one. If you can solve this problem, name your price in the proposal. Here is the upwork project listing:

Upwork job: Access denied.