I bought a refurbished S4 Mini a few years back, and aside from the fact that it won't allow tethering (it was originally an AT&T handset) it's been OK.  However, it stopped working at our bach around a year ago.  The bach (Taupo Bay, Northland) wasn't serviced by any mobile telcos until a few years back when Vodafone put a cell in.  I was able to piggy-back and we ditched the land-line.  Spark acted pretty quickly and provided service for their customers as well, and soon 2degrees had their own service there also.

2degrees support couldn't really help, but said the timing suggested it might be to do with 2G support being phased out.

When I try to select the carrier manually, only Vodafone and Spark show up, and it won't let me connect to Vodafone.