On the LAN Interface, change the default IP to a different one within the target subnet and disable the DHCP server.
For a wireless interface (working on the same frequency band as the Access Point), click Scan, join the previously created wireless network and when asked, set the firewall zone to lan.
The wireless mode should be Client (WDS) and the Network in Interface Configuration has to be changed from wwan to lan.
Go to Network, DHCP and DNS. Set DNS forwardings to the ip address of the AP router.
Go to Network, Interfaces, Lan, Edit. Set IPv4 gateway to the ip address of the AP router.
Go to Network, Interfaces, Devices tab, Configure… on br-lan, Advanced device options and enable STP. Failing to do so can allow a network loop to form that will take down all routers.
Finally, add a new wifi network if you want to join the network via wifi. It can have the same name, password and settings than the WDS access point, or they can be different. When creating the new wifi network, under General Setup, ensure that Mode is Access Point and Network is set to Lan