The only way I’ve got mobile dongles to work is by by-passing connman and ofono, but it would be good to get them supported with connman and so that you don’t have to switch off eth/wlan as described by @balnafasae above.
There’s a feature request on this. When I last looked, I couldn’t get connman to see either of two 3G devices I tried and the connman documentation wasn’t much help.