Can't get the 2560 x 1440 pixels resolution on an external monitor connected to a Macbook Pro 15' Early 2011

Hi there,

I really hope someone has the answer since I visited 3 different electronic stores (Apple included) and have been spending many hours googling. Here is my problem:

I just bought a new AOC Q3279VWF monitor, but I can't get my macbook to display the 2560 x 1440 resolution on my external monitor.

What I know:

* My macbook pro can display the resolution on an external display that I want, since I looked it up and I can used an Imac with that resolution as external monitor.

* My monitor is not broken since I connected a newer macbook pro with the same cables to my external monitor, and it did display the resolution I wanted.

* My cable is thereby also not the problem and I also tried 2 different cables:

- HDMI (included with the monitor) on an thunderbolt mini displayport to HDMI adapter.

- I bought this belkin cable Belkin 4K Mini DisplayPort to HDMI Cable.

What I tried

* Holding down the option key when in the display settings which displays the other possible resolutions.

* I reseted the PRAM.

* I reseted the display.

* I downloaded SwitchXRes, but I can't add a custom resolution.

Does somebody know what can be a possible solution?

Maybe I need to connect a different cable to my external monitor? The possible connections to my external monitor are HDMI, DVI, VGA and Displayport.

Thanks in advance!