Open the online S2716DG User's Guide to page 30. We see that Input Source gives you the option of adding a check to either HDMI or DP. Did you do this for HDMI? If yes, then turning the S2716DG off must be clearing it. If so, consider leaving the S2716DG on but allow it to go into PSM (Power Save Mode) via the operating system power settings.
The Dell Community Forum is designed to be primarily a user to user troubleshooting interaction. The purchased warranty on your S2716DG (built on September 10, 2015) expired on November 18, 2018. The DELL-Cares agents can only assist users who have an in warranty Dell product. As a courtesy, I did post some advice on your public thread.