The fallback mode is an ordinal list of ways to connect the device to a network: LAN, wifi or GPRS (SIM); and in what order you want to use them.
You can change that order to the preferred connection in which you want to connect to a network when you turn on the terminal.
If for example you have LAN - WIFI - GPRS, and you take out the LAN cable, after a reboot it will connect to the wifi, if you configured the password. If not, it will start the GPRS, if you have a SIM in the terminal.