How can I get the signal back on my google nexus 4?

[greymbw]

Due to an international trip I haven't used my phone for 2 weeks. I tried installing an Irish simcard and this is where the problem started. I couldn't get a signal. Thought perhaps it was to do with the phone not having 4G capability in a 4G network. Then on return to Sydney Australia, I have tried rebooting to safe mode and checking the APN settings.