ogxun no matter how you installed Flarum, you should still make sure your server meets all the requirement to make the best out of it, in particular Composer and SSH access. With Composer you can then install extensions using the commands described in an extension discussion or README file.
If you don't have SSH access to the server, this will make your life more difficult, but it can be done https://docs.flarum.org/composer#i-dont-have-ssh-access
There's also our package manager extension in beta that you can find on this forum, but it's not ideal right now if you don't have SSH access as you might get stuck in case you encounter a bug.
