ZyWall USG-100 Firewall block access to internal subnet beyond another router

Question

I'm experiencing a strange problem with a ZyWall USG 100 Firewall, that I can't solve with leaving the firewall enabled on the ZyWall.

I have the main network subnet that is 192.40.192.0/24(LAN1 on the ZyWall) and an access point that allows connection to this network to wifi clients with a main router firewall on IP 192.40.192.1 (ZyWall USG 100) that serves as DHCP client too. Then I have another subnet 192.168.100.0/24 that is behind another router(IP : 192.40.192.254) that is a Linux server. The problem is that if I want to access to a web server on the secondary subnet, for example 192.168.100.200 the HTTP connection is blocked by the ZyWall Firewall to wifi clients .

If I disable the Firewall on the Zywall or if I add a static route to a wifi client, like this :

sudo route -n add 192.168.100.0/24 192.40.192.254

everything works correctly as it works correctly too with a client directly connected to an Ethernet LAN port.

I don't understand why the firewall seems to block the connection since I have a rule like this that should allows all the connections :

Is there something am I missing here in the configuration ? Do I need to add some rule to the Firewall ?

Why it happens to wi-fi clients only ? What could it be the reason ?