HOW TO: Force all logins for subsites to go througth the main site login

Hello,

I am looking to force all users of subsites, to use the main site login, then redirect to their correct dashboard.

For example, if a user tries to login on the sub-site.com/wp-login, they are forced to login via the main-site.com/wp-login.

There is a chat over here about it, and a MOD posted a plugin from 2013 : https://premium.wpmudev.org/forums/topic/i-want-to-prevent-anyone-from-logging-in-to-subsites

Is there a simple way to do this?

My setup is:

WP 4.7.3 – Subdomains

WPMU – Domain Mapping