Brief: SonicWALL IP Spoof on WAN from Similar Subnet.
While this article seems like the resolution doing what it detailed did not resolve the issue:
I have a Unifi Controller behind a SonicWALL.
We have multiple sites we control from it.
If the site is on a static IP from the same ISP (only 2 ISPs in town) and has the same first 3 octets the traffic passes fine.
Server site WAN IP: 18.104.22.168
Client site WAN IP: 22.214.171.124
However if a site is on a different octet then they cannot communicate due to "IP Spoofing" detection.
Servers site WAN IP: 126.96.36.199
Client site WAN IP: 188.8.131.52
I've talked with SonicWALL and their engineers are working to find a resolution but I don't know if they can come up with anything.
The server site ISP WAN IP is a /30 net mask.