I paid for a factory unlocked iphone 5C from Sprint and I have tried to use it on another carrier. The answer came up as SIM NOT VALID. Can someone help me.

I have been trying to use a factory unlocked iphone 5C i had bought for me from Sprint,

I cannot understand why when i put another sim card in, it said SIM Not Valid (as in the photo).

I tried again and again and nothing happened. But this sim card works fine in my iphone 4S.

I then paid for another sim card from t-mobile and that was 'new' - but it didn't work.

Can someone PLEASEEEEEEEE give me some advice as the phone was bought for me by my grandpa and i am now away traveling and get it to work. Furthermore, I am not sure if he bought it directly from Sprint. But the carrier says that name. If it is unlocked why can't i use it, even abroad?

HELP.

I