Do you mean you had the phone unlocked? You don't unlock SIM cards as they are specific for each carrier. If you did not unlock the phone then it's still locked to Koodo. They don't, however, relock phones after the fact.
You need to get a new SIM card from a compatible US provider.
Normally unlocking the phone from KOODO involves paying for an unlock code which is specific to your particular phone's IMEI ("serial number").
You then insert a SIM card from your alternate US provider and when the phone boots it asks for your unlock code. You punch it in and all should be well. May have to call 800 number to get data working, but calls and texting should working instantly.
The SIM card itself is never affected by the unlock process --- only the phone electronics.
In the US, you have to use a carrier that is AT&T compatible for your hardware to work "out-of-the-box". Try Consumer Cellular. SIM cards available at Target and Sears stores for $10, which they will refund once you're up and running. Their customer service is US based so at least you can understand what they're saying.