Static Routing issue

[SimpleRezo]

Hi !

I am facing with trying to set up a static route with OPNSense.

OPNSense has a WAN connected to a modem.
LAN is 192.168.9.0/24 and on the LAN i need to use 192.168.9.1 as gateway for network 192.168.254.0/24.

Here is that I have done:
  - add a "Single Gateway" / LAN / 192.168.9.1
  - add a route using that single gateway for 192.168.254.0/24
BUT if i enable the new gateway, LAN is loosing internet connectivity and OPNSense is using this gateway as default route (check by netstat -rn), despite is not marked "default" and the other gateway is of course still marked as Enabled AND Default (and setup for interface WAN).

So, if :
  - I boot with the new gateway disabled, the route is not present ;
  - If I enable the gateway, the route is added, BUT my LAN is loosing Internet connectivity ;
  - If I disable the gateway, the route is still present and working (and LAN has Internet connectivity) !

More (maybe) useful information:
  - Gateway switching is Enabled (System/Settings/General)
  - Interfaces / LAN / Upstream Gateway set to "auto-detect"
  - Interfaces / WAN / Upstream Gateway set to "GW_WAN"

Thanks for your help!