First, you have not call nathelper at all. It's not in your logic. So, no NAT transversal will happen.
if (method == "REGISTER" ){ log("LOG: Someone trying to register from
private IP, rewriting\n");
fix_nated_contact(); # Rewrite contact if (method == "INVITE") {
Here, you are checking INVITE within REGISTER request. This is not possible. The fix_nated_sdp() will not run at all.
fix_nated_sdp("1"); # Add
direction=active to SDP };
force_rport();
Search the archive for examples on how to work with nathelper and rtpproxy.