After installing auto-usb-wann, a hotplug.d script will detect when any USB WiFi device is connected, automatically configure it to connect as client to a predefined SSID/password and ask for DHCP. The idea is to provide internet access to a router, without needing to mess with LuCI or CLI. Just insert a supported USB device on the router, and open an AP with 3g/4g tethering on a phone.
By default, it will try connect to SSID "internet" with password "internet", you can edit this configuration in /etc/hotplug.d/ieee80211/11_auto-usb-wwan