IPhone locked to Fido, yet doesn't work with Fido SIM?

I broke my samsung phone, so my brother is letting me use his old iPhone 5, and I went to Fido (the company we've been using for years and both our phones have been with them) and I bought a SIM card for it and activated it, but when I put it into the iPhone, it says that it's an invalid SIM stating that the iPhone is locked to another carrier which if it is it wouldn't matter because it's the same carrier.

Sdfgsdfg

Did you make sure to get the right SIM for your iPhone when it comes to pre-paid/plan and post-paid? They have SIMs that will be the same, but authentication being different from each other.