D-Link DSL-2750U Technical Support Setup Procedure page 16

-16-
Note: To achieve WAN fail over, 3G connection will be triggered to connect
automatically when ADSL connection is not available.
Please Do NOT click on the "Manual Dial" button. Clicking on "Manual Dial" button will
force the 3G connection to be used as the primary internet connection, and WAN failover
function will NOT work!
By default, when there is an ADSL connection and a 3G connection configured on the DSL-2750U, it
will take the ADSL connection as the PRIMARY WAN connection.
If ADSL connection is up and running, 3G connection will be in „disconnected‟ status. It will standby
for WAN Fail Over.
FAQ : How To Configure 3G WAN Fail Over