AW2518HF, support, 240Hz

Hello,

I am having issues when using 240Hz. It goes black screen for 1 -2 seconds. I was using 144Hz for months without any issues. Also I've tried several other cables. This problem is present with both of my graphic cards AMD RX 480 and RX 5700 XT. Now I'm trying to find technical support but there isn't any for this product. When I'm using "search" it doesn't show any result.

Any help much appreciated