Is it possible that someone is able to open it through WiFi???
No, unless they know your Passcode, Apple ID and Apple ID Password.
When you set up Lost Mode on the phone, the action is "Pending" until the phone has a good Internet connection. When the phone has a good connection, Lost Mode will be activated.
can someone still open it and use it???
No, unless they know your Passcode, Apple ID and Apple ID Password.
If you are still concerned, my recommendation would be to Erase the data on the phone. You will never see the phone again anyway.
Be extremely careful if you see a message from "Apple" or "Apple Support" that your phone has been found and you just need to provide more information so the phone can be returned to you.
Apple does NOT get involved with lost or stolen phones, so any message like this that you might see is a scam that is trying to trick you into providing your personal details, so the bad guys can get into your phone.