I have succesfully enabled it in /etc/yum.repos.d/fedora-modular.repo and /etc/yum.repos.d/fedora-updates-modular.repo by changing enabled=0 enabled=1 under [fedora-modular].
It works but i would also like to know if it is disabled for a good reason.
Edit: The modular repos can be enabled from Gnome Software’s “Software Repositories” menu now.