The problem isn't the HDMI-cable or HDMI-port, as it worked before. I sort of found an solution: I disabled the driver for the display adapter on my laptop, and now it works 1920x1080. Don't know why, but hey, it works.
When I try to use my Inspiron 7790 AIO as external monitor for my laptop, since a few days the maximum resolution is 1360x768. I did not change anything in the settings and when I connect my laptop to another monitor, I get a higher resolution. How can I solve this?
Using Inspirion 7790 AIO as external monitor - resolution problem