I can receive but not send messages on LINE APP when connected to WIFI. I have to disconnect from the WIFI and reconnect for it to go through (in which I believe it's using 4G to send).
According to Line, it should use either one, but you can email them at line-support@line.me and ask them for help. (If it uses wifi, there's no reason for them to add extra code to detect which internet connection you're using, so there may be a bug in the app, or there may be some incompatibility with your particular model phone.)