Make sure you are not behind a Symmetric NAT. If so, you're dead. STUN does not work with Symmetric NAT.
If a UA is behind Symmetric NAT, and UA use STUN, and SER have [RTP/Media]Proxy to handle Symmetric NAT, this UA should be fine, right?
Yes, but, if UA is behind symmetric NAT, I would not configure STUN to it. I'd just led mediaproxy solve the problem.
Regards,
Lucas