DELL UltraSharp U2515H monitor doesn't recognize Macbook Pro (early 2011, OS Sierra)

Hi guys!

I have an issue with a DELL UltraSharp U2515H monitor: it doesn't recognize my Macbook Pro (early 2011, OS Sierra).

I have tried different cables, inputs, adapters..

When I plug the cable into my MBP, my MBP recognizes the display but on the display itself it says: "There's no signal coming from your computer." and then it goes into rest mode.

The display did work with a later MBP of a friend (don't know which one exactly) with a DisplayPort to Mini DisplayPort cable, but it wouldn't work with the exact same cable and my MBP.

I have since then tried it with two different HDMI cables connected to a HDMI to Thunderbolt adapter - to no avail. And I really don't think the adapter is the issue, since it has worked flawlessly before and my MBP does recognize the external monitor, just not the other way around.

On DELL forums/website it says that the monitors are not specifically developed for and tested with Apple products, so I might just be unlucky with having this (old) MBP?

The display works fine with my PS4 (that's how I know the display and the different HDMI cables are fine).

Help would be much appreciated!!