External display of 2560x1600 maxes out to 1920x1200

[jorbig]

External displays with a 4K resolution are supported just fine over USB-C, but not displays with 2560x1600, which max out to 1920x1200. I tried manually editing the monitor.inf as suggested here, but I can't install the edited .inf, since it complains about the driver not being signed. Somehow turning off testsigning didn't help. Any ideas for a possible solution?