iMessage issues

My iPhone was recently factory unlocked because it was locked to a network in Japan. After said process, I inserted my sim card and can now receive calls and messages but I can’t send out messages. My phone number is not listed in iMessage only my apple id. I’ve visited numerous sites and done all they asked me to do including resetting my iPhone. Even after, no change. I am unable to contact Apple Support so I’m asking for help here. Please help.