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.