Posting on behalf but I'm trying to assist with a phone that was used in Australia and Vietnam, originally it was set up with roaming prior to departure, it worked briefly in Melbourne but stopped not long after arrival. 
After many hours of back and forth it's believed a Vietnamese esim was purchased once in Vietnam.  Now back in NZ the NZ esim doesn't seem to be able to be loaded back onto the phone or made to work with the phone and the phone is only able to connect to wifi.   

The phone is a Samsung Galaxy S23 Ultra.

Spark have tried to figure it out but have given the advice of running it over with a car to claim insurance which isn't an option.  

I don't have the phone physically and I'm not sure what settings are what on it, but I can get more information or happy to receive advice on where to go with it.