iPhone carrier/international sim

I Purchased an iPhone 5c directly from an apple store last year. I purchased the phone and provided my own SIM card from Telstra.

I cancelled my telstra account and purchased an international SIM card from ekit (ekit.com).

i Put the sim in my phone in aus and it connected to the Vodafone network. Upon arriving in chile it connected to Movistar and claro networks in every location. In Bolivia I also connected to both these networks and sane for Peru with no problems.

however after 3 weeks I was suddenly unable to connect to any network. I figured this was due to my remote location, but when I arrived in Lima perus capital I still cannot connect to any network. since then I have visited Ecaudor and Spain and have not connected to any network since.

i Contacted ekit and they suggested manually choosing a carrier (settings->carrier etc.) but in settings I do not appear to have a carrier option (I checked someone else's phone and they had it)

EKit suggested this is because my phone is locked.

My question is, how can my phone be locked if I purchased it outright from apple.

why would my phone initially