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:
- Select 2G on the non-LTE-card
- Select 4G/3G/2G on the LTE-card
Only one card is able to use LTE. So the other card has to be configured with 2G first. That's the trick!