How to fix S8 not getting texts anymore?

I've recently got the S8 two weeks ago in a trade in deal from Best Buy with Verizon. I traded it in for an iPhone. When I first tried to text my friend with an iPhone, the messages would still go to the iPhone and not the S8. I fixed the problem by disconnecting my iMessage. However as of last night when texting my friend, it immediately stopped working. I can send messages and they can receive them but they sent messages and I don't receive them. What do I do? Is it because of the recent update to the S8?

Make sure the sim is transferred properly since they use different size SIM cards.

"I traded it in for an iPhone"…

You're not making sense. Do you have an S8, or an iPhone?