Check and see if there is a setting in your firewall for consistent nat. it's typically in the VOIP area of the firewall. when you are doing voice communications you want to make sure that the port mapping stay consistent. That's probably part of your problem.
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Thanks for the suggestion, this led me to the correct area and I finally was able to resolve this and get an Open NAT without using UPnP. On my router I had to give my PC a Static Port in the Outbound NAT configuration which allows the external STUN Port to always be 3097. Once this was done the forwarding rule I had setup previously worked perfectly and it now shows as NAT Type Open.