Just guessing, but could it have to do with
- APN settings (see this thread: https://fairphone.zendesk.com/hc/communities/public/questions/200670358-Anyone-having-trouble-using-mobile-data-network- ), or
- the fact that the FP (as, apparently, all Android phones) can’t have 3G activated on both SIMs at the same time (see Enable 3g on both sims? )?