Just tried a factory reset. Same issue. I have tried VOXI, Vodaphone vanilla and Giffgaff (o2) in the router from working phones. Same issue.
Check if your APN field is not set to manual/custom with the wrong APN selected or defined. Checked and its on auto.
Make sure your operator didn't do any changes to their side of the system. Maybe your IMEI got blacklisted or wasn't whitelisted when something was happening on their side. I contacted smarty and they said the IMEI is not blocked.
Check if your frequency bands are set to automatic. Its set to auto