[Serusers] UA, SER, RTPPROXY in local net, some external clients with public ip

Hello, serusers.

I am newbie for SIP. I installed ser, rtpproxy and read manual very many times :-), but...

Ser server and some clients in local network : ser - 192.168.100.1 UA1 - 192.168.100.2 UA2 - 192.168.100.3

Some clients have public ip address, not behind firewall...

All work fine between local clients, but not work with external clients. I think that local clients register with local ip in Contact field from local network and cannot be fix with fix_nated_ function, and so can't access from external...

Simple config or config for rtpproxy from gettingstart not help me... Can anybody help me with simple config for this network configuration?