1. Add a New AP Access Point
Go to Network –> Wireless page and click the add button for your wireless interface.
Create a new network interface in the network section (other settings can be customized).
2. Edit the New LAN Interface
After saving and applying, a new interface will appear in Network –> Interfaces, click edit.
Switch its protocol to static address and set an IP for it.
Create a new firewall rule in the firewall section.
In the DHCP server section, click the configure DHCP server button.
3. Modify Firewall Configuration
Finally, go to the firewall section, edit the newly added firewall, and change input, output, forward to accept, accept, reject respectively, and set Allow forward to destination to your WAN.