[Serusers] UAs and SER behind NAT

Hello, I'm sorry for this basic question, but I have SER running on my LAN with (RFC1918) addresses with UAs on the same network. Then there are UAs registering from behind NAT to this SER on the LAN network. (SER is not listening on any public IP and it's just used in the private network) I came across this issue. If a UA on the LAN network is registering to SER the logic of ser.cfg recognize the UA is behind NAT after nat_uac_test. If I want to introduce NAT traversal with RTPproxy for this private network nat_uac_test will always return that the UA is behind NAT because it has RFC1918 IP address. The RTPproxy will be used at all times for UAs on the network or even I introduce STUN for NATed UAs I know this is not a common usage of SER because it is invented to be used on the global internet but I was just wondering is there any possibility not to use RTPproxy at all times for UAs on the private network? Maybe this is a silly question but it would help me for my testing purposes with SER where I will not have to use my setup with usage of public IPs. Thank you for any thoughts. Ladislav