This is not the phone playing up but a change in cellular networks (also called the cell carrier) moving to 3G and up.
Since you were able to buy them in Aus, then that's as good as released but I don't expect them to work.
https://www.nperf.com/en/map/AU/-/2445.Telstra/signal/?ll=-34.07710207862542&lg=145.04076171874996&zoom=6 shows no 2G coverage by 3 carriers.

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