SIP peer Registered Contact with correct IP but being changed to PBX IP (Addr->IP)

I am using Asterisk 13 and facing a puzzling problem.

All remote users connecting to this Asterisk box are presenting the correct “Reg. Contact” with their source IP, but the Addr->IP field is switching it to the IP of this Asterisk box. All remote peer registrations and calls are progressing as normal. The only challenge with this is - Fail2Ban is blocking all remote users enmasse since all remote connections, including probable hack attempts, are all reflecting “THIS.IS.ASTERISK.IP” instead of their respective “THIS.IS.SOURCE.IP”. Can anyone throw some light on why this may be happening? Could this be resulting from the way the edge router is passing packets to the Asterisk box? Appreciate any pointers. Thanks.

Defaddr->IP : (null)
Prim.Transp. : UDP
Allowed.Trsp : UDP,TLS
Def. Username: 202
SIP Options : (none)
Codecs : (ulaw|alaw|g729|vp8|h264|h263|h263p)
Auto-Framing : Yes
Status : OK (13 ms)
Useragent : Grandstream GXP1615
Reg. Contact : sip:202@THIS.IS.SOURCE.IP:1033