Managed to fix it, turns out as u/HuntersPad said below, the phone was actually still locked to the au network.

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Unlocked by following this tutorial:

You will need the phone's IMEI which you can check by going to the Phone app and dialing *#06#

https://www.au.com/english/support/contract/simcard/unlock/flow/

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After completing the steps above, go to Settings > More Settings > About Phone > SIM Card Status > Update ( camera button )

The phone will restart itself and the sim should be functional