D-Link DSL-2750E Technical Support Setup Procedure page 11

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Step 5: Scroll down the page.
Under "Backup Mechanism", please select "DSL". The router will check on DSL line and Ethernet WAN
status. It will trigger 3G connection when both DSL and Ethernet WAN status is down.
Alternatively, you may configure the 'Backup Mechanism' as "IP Check". In this mode, the router will
do IP address ping check periodically. If the ping is fail within the defined parameters, 3G connection
will be triggered. You may enter your own prefer IP address for checking and period of check.
Click "Apply" button.
Step 6: You may see the 3G connection Status showed "Connected" as following.
You may now access internet via 3G connection. The 3G connection will be disconnected automatically
once the primary WAN (ADSL/Ethernet WAN) is back alive.
FAQ : How to Configure 3G Fail-over on DSL-2750E