sip_nat.conf file is a file used by GUI system like freepbx or elastix, a sip trace would be helpful or rtp debug. Also remmber on the new channel driver pjsip you have the options
external_media_address=External IP address to use in RTP handling
external_signaling_address=External address for SIP signalling
this can be added indivually for each trunk
Also you could try to conect your CLARO gateway to a router with NAT support, and then add that router to you Asterisk network, that will help you to avoid to use 2 network cards and posibly fix the issue if router and asterisk nat setting are adjusted properly.
Also using 2 Asterisk servers it is good deal