E7470, E1715S, no signal from Latitude

Recently found a 6 year old Dell E1715S, manufactured September 2017) monitor in used condition from Goodwill. I have connected it to a 6 year old Dell notebook model E7470 (bought in 2017 October) via Mini DisplayPort to DisplayPort cable (the cable didn’t came supplied). However, the screen says "There is no signal coming from your computer. Press any key on the keyboard or move the mouse to wake it up." Doing so doesn't do anything. 

Cables are all firmly attached and plugged in, have unplugged and reattached several times to be sure. Holding down buttons on monitor for 20/30 seconds+ does nothing, only produces padlock symbol and then "entering power save mode." Have tried downloading drivers but installation setup indicates that no monitor is connected. Don't think there's an issue with USB ports as these connect to keyboard and mouse fine.

Any recommendations gratefully received. Laptop is difficult to disassemble but can try doing so as last resort. Was planning on using monitor as additional screen alongside laptop.