so apparently you can only add new content types when in development mode because of the way nodejs apps work at least that is what i was explained on github for a similar question.
I have my app running on ubuntu 18.04 on vultr.com. Strapi is running always with pm2. Whenever i need to add a content type i would just stop pm2 and then run yarn develop and then once i’m done i just start pm2 again.