In these two threads we seem to see that FP3 is susceptible to loosing #apn at an update.
In case it is actually a more simple carrier issue you might experiment with
- Power down, remove SIM card, (clean) and reboot.
- Settings > Network and Internet > Mobile network > Preferred network type – experiment with an alternative type of connection.
Edit … and rather than doing APN the hard way … just call the carrier and ask them to resend the APN to the device.