Fortinet automatically detects and use IP as DHCP of another router

[bootstrapper]

Device: Fortiwifi 30E

We have 2 locations that is connected together via P2P using Ubiquiti PBE-2AC-400.

Location A - Main Office use Fortiwifi 30E with it's own internet connection

Location B use TP-Link Archer D20 with it's own internet connection - Different IP Address. Now we have RDP's to Location B which is successfully running using our P2P connection and just adding secondary IP (IP range from main office) to client PC's (Location B).

Our problem is every time we connect Location B to Location A. Location A (Fortiwifi 30E) automatically detects the IP address of the TP-LINK Router on location B and using location B's IP address range as it's DHCP.

Question is How do we stop our fortinet device at main office from automatically detecting and using the IP address of Location B please?

Not a network wizard so I am bumping into a wall here.

[Toshi_Esumi · Accepted solution]

Sounds like you have a bad broadcast domain design both and between two sites. I'll recommend you split it to sections and the P2P circuit is terminated at a L3 device Like FortiGate and assign /30 subnet then route, not bridge, all traffic between the locations. And leave both sides have different IP Subnets, not overlapping each others.

Otherwise, even if you could patch the FGT for the immediate problem, you would soon encounter different problems one after another....never ending nightmare.