Lately i have changed for an LTE contract with 1&1. I used the SIM card in an old iPhone 4 and had mobile internet ( G3 only of course).
On FP2 I have phone but no data. While the congstar prepaid shows “3G” next to the connection quality symbol. There is nothing like that at the simbol of the 1&1 LTE Sim, even when it Is the only inserted.
If you use two SIM cards in the phone, please keep in mind that the Fairphone 2 will only support 3G/4G for one SIM card (hardware limitation).
You have to explicitly set one SIM card to 3G/4G in the SIM settings, the other one will be limited to 2G. To swap the setting between the cards, as far as I recall you will first have to demote the current 3G/4G card to 2G, then set the other card to 3G/4G.
If you get no mobile data at all, not even slow, then please check the APN settings with your provider.
What do you find at Settings -> Wireless & Network: More -> Mobile networks -> Access point names? If there is no APN, you might have to configure one.
I think you can only use one SIM-Card-Slot for 4G at the same time. You can switch the Network Type for every Slot. If one of them is set to 4G, then it’s not possible for the other one.
So if You want to use the other slot for 4G, You must first set them both to 2G (bevorzugter Netzwerktyp) and then you can set one of them SIM to 4G. So both of the slots should work with 4G, but not at the same time.