An iPhone 4 I got in March 2011 in Australia was able to be used with a SIM when I visited Japan in 2011 to 2013. However, when I tried getting a SIM for it in September 2016, the store staff said that that phone was too old to use one of their SIMs.
Is that true, and if so, is it something I can fix for that iPhone, such as upgrading iOS version? It uses a micro SIM, has a model number of MC603X/A (corresponding to iPhone 4), and has a version of 7.1.2 (11D257) which seems to indicate the iOS version.
I'll be going to Japan and Korea this September, and I've got a more recent iPhone as well, but want to take this as a backup.