Registration Issues - Incorrect Remote Address

Hi guys,

I am using FreePbx 2.11.038 and I am having a strange issue that I am trying to trackdown.
To give you some background we have users who have a Thomson Router which connects to an Ubiquiti Antenna, the Antenna provides the PPPoE connection and the router connects to the Antenna passing the connection through. The thomson is the device where the voip settings are configured.
When setting the devices up I am able to configure the Voip settings and the phone successfully registers and calls can be made as expected.

The issue comes into play if there is a loss of connection. If the user has a public IP of say 80.121.121.1 and then the internet cuts and they are reassigned a new IP eg 81.141.141.1, the registration is lost.
I have tried rebooting freepbx, the router and the antenna and it seems the only thing that makes the difference is rebooting the antenna. Until this is done, registration will not occur and continues to fail.

When looking at the output of asterisk -rvv I noticed the following line

CLI> ^M^[[0K[2014-11-25 12:07:47] ^[[1;34mSECURITY^[[0m[1671]: ^[[1;37mres_security_log.c^[[0m:^[[1;37m134^[[0m ^[[1;37msecurity_event_cb^[[0m: SecurityEvent="ChallengeSent",EventTV="1416913667-75672",Severity="Informational",Service="SIP",EventVersion="1",AccountID="100167",SessionID="0x7f627c05e218",LocalAddress="IPV4/UDP/thepbxip/5060",RemoteAddress="IPV4/UDP/80.121.121.1/5060",Challenge="66bca268"

It seems that although the IP of the connection has changed from 80.121.121.1 to 81.141.141.1, the old IP (80.121.121.1) is still being declared as the RemoteAddress. I can only presume that obviously when say the ACK or similar is returned to this address it is obviously going to the incorrect IP. Has anyone come across this issue before and does anyone have an idea of how it could be resolved.