iMessage question

Hello,

My wife and I were using imessage with no problem here in Canada until I took a trip to the US.
When I crossed the US border I put a T-Mobile sim card in my unlocked iphone 5 and and everything was green after that...basically no more imessages. My T-Mobile daily plan has unlimited data.
My data roaming was turned off and someone told me that it should have been turned on ??
I'm home now and facing a a bunch of charges for international texting and just wondering if I did wrong and why the imessage didn't work while I was in the US.:confused:

Thanks,Dave