D-Link DSL-2878 User Manual page 40

Section 4 - Configuration
Connect mode Set the connection to be Always-on, Connect-On-Demand, or
select: Manual.
If you selected Connect-On-Demand, the following option is available:
Maximum Idle Enter the amount of time the router will maintain the Internet
Time:
connection before disconnecting if there is no activity.
Click Save when you are done.
D-Link DSL-2878 User Manual 36