You can temporarily disable the firewall via command line by executing this command:
To stop: /etc/init.d/firewall stop
To start: /etc/init.d/firewall start
However, this is purely temporary and the firewall will start back up again after you change any network related settings or reboot the router. For a more permanent solution you can add the following line into the Custom Rules section (reached from the router's WebUI, Network → Firewall → Custom Rules page):
This command will 'flush' all firewall rules and it will do so each time the firewall is restarted (after network setting changes, router reboot, etc.) effectively making this a permanent solution.
Important note: before disabling the firewall, make sure that you know what you're doing and why. The firewall is there for a reason, disabling it will leave the router and its network vulnerable.