This device from Asus does not have a firewall which masks the contact address of SIP packets. This means that SIP traffic and RTP traffic pass through without interference and should work with OnSIP without issue.
If there is a SIP ALG running, telnet into the device and enter the following commands:
nvram get nf_sip (It should return a "1") nvram set nf_sip=0 nvram commit
Then reboot the router for the changes to take effect.
Certain firmware can cause disabling SIP ALG to stop all SIP traffic. It is a bug in the router software. This is the case with their firmware version 188.8.131.52.380.7378. With the latest firmware, it's recommended to leave SIP ALG enabled.
For phones to pick up the change immediately, reboot each of them, otherwise they will pick up the new NAT table with changes during their next registration.
Optimize your network for business VoIP with the right router