D-Link DSL-2790U User Manual page 61

Configuration
This window allows you to set up 3G Failover on the Router. When
you are finished, click on the Save/Apply button.
Quality of Service
QoS or Quality of Service allows your Router to help prioritize the data packet flow in your Router and network. This is very important for time sensitive
applications such as VoIP where it may help prevent dropped calls. Large amounts of non-critical data can be scaled so as not to affect these prioritized sensitive
real-time programs.