It really depends how you configure and use your Linksys device.
Standard implementation supposes that Linksys WAN interface is connected to Zentyal and devices connect to AP like if connecting to Linksys LAN.
In such case, on Linksys, IP on WAN interface is known by Zentyal but, depending on Lynksys conf, IP on Linksys LAN and Wifi might not be known.
You may have to add static route.
Another approach is indeed to use Zentyal DHCP (which I would suggest, not because this is better but because having, on same LAN, all DHCP configuration at same place eases management).
In such case, you will have to configure DHCP relay on Linksys.
Notice there is no need to change your netmask as you suggest in order to get connectivity.