Today I inserted another sim card in my fairphone 2. I want to use LTE with this card, but I wasn't able to configure 4G/3G/2G. 2G was selected and the button was disabled. So I just had 2G. The other card worked as expected and I was able to choose 4G/3G/2G. This bothered me the whole day.
In the evening I figured out how to fix the problem: