Hello railmond124,
Thanks for reaching out to us in Apple Support Communities about activating your iPhone with Boost Mobile, we're happy to help by looking into this more!
There are a few reasons why you might have issues with activating your iPhone. You first need to make sure that the iPhone doesn't have Activation Lock:
Activation Lock for iPhone, iPad, and iPod touch
Only the person that enabled Activation Lock on the iPhone can remove it. Another potential issue is if the iPhone is currently locked to a carrier in Mexico, that carrier would need to release the iPhone:
How to unlock your iPhone for use with a different carrier
You will need to make sure that Boost Mobile is compatible with this particular iPhone. Boot mobile would be able to confirm this for you. Some iPhone models are designed to only work with particular cellular networks.
Hope this helps.
Best Regards.