Thanks for the reply! It's on the latest firmware (6.04.5). Interface was set to automatic.
I deleted the config and made a new one, with interface on automatic this time too. Still the same, restarts every time after retries.
So I tried to set interface manually, the only other option being "Ping from mobile". When I choose that and enter 184.108.40.206 into both SIM1 and 2 text boxes it says "One or more required fields have no value!" and I can't save the settings. Now I'm really confusd what I'm doing wrong... :-)