Go to Network > Wifi and remove the current interface:
From Network > Wifi, add a new wireless interface:
The next screen will present a list of the available wireless networks which are in range. Click Join Network next to the wireless network you want to join:
Enter the passphrase of the Access Point, if needed, and click Submit. And then click Save & Apply on the next screen:
After that, the router should have joined the wireless network. From Network > Wifi, it is possible to check the status of the wireless interface and the associated stations:
From Status > Overview, it is also possible to check that the router should have a WWAN IP now and the interface should be marked as “online” with a green background.