No contact does not mean ignore your ex. It means don’t contact your ex. Let him contact you. When he contacts you, you should respond, but keep the response casual so you don’t come off as needy and desperate. Your ex reaching out to you is usually the goal of no contact. So congratulations.
It sounds like you got some bad advice somewhere and what you are actually doing is “playing hard to get”. THAT could hurt your chances of getting your ex back, as he will only try a couple times before getting the hint.
If there is a message in the snapchat, answer the message. If it is just a picture, just respond with “Hi”. If you aren’t sure how to respond, share more info here and I will help you.