WireGuard Server gives no access to specific LAN (static with no DHCP)

I’m in trouble with a non standard configuration, and too short in network knowledge.
Please, could you give me a help ?
My device is a Mango “GL.iNet GL-MT300N-V2”.

Its LAN is connected to an old local network that I can’t modify : all computers have a static IP and there is no DHCP.
Thus the IPs of my Mango :

  • WAN: → static delivered by the main router, which gives access to the WEB
  • LAN: → static defined via Luci, no DHCP
  • WireGuard: port 51821 → attributes modified via ssh: vi /etc/config/wireguard_server (to distinct from a previous Mango)

In Mango > WireGuard Server > Local Access is ON.
My remote WireGuard clients can reach WAN side but can’t reach LAN.
I’m a beginner in routing and don’t know how to handle the problem.
Could you give me a help ? Thanks in advance.