What make/model of cable did the shop sell you? How do you mean it doesn't work? It should auto default to using the DP, but you may need to change the source on your monitor as it may be on HDMI. If everything is as it should be, I'd take it back for a refund or have a tech savvy friend come over & configure it.
Here's an example of a cable that would work:
https://www.amazon.com/dp/B078HVDMW2/ref=psdc_3236443011_t1_B01J8S6X2I
Plug it into the DP on your 1070 and your monitor.
Open your NVIDIA Control Panel and click on the Change Resolution tab on the left. A screen will open. Look at #2 and it'll show how your PC is connected to your monitor. Choose your resolution and the hz.
Let us know when you get it up & running...