my iPhone 6 is locked in my country

I bought an iPhone 6 on February at T-Mobile, I asked them if the iPhone was unlocked to be used in a foreign country and they said that the iPhone was going to be unlocked when I finish paying it on August but I had a problem with that iPhone on July so I went to an Apple store and they changed it for a new one. I'm not in Usa now and I don't know what to do because when I came to Ecuador I wanted to use my cellphone and I tried with 2 SIM cards from my country but they didn't work because on the screen said sim not valid. Could anyone help me please?