Hello, @tim-leal,
To set the maintenance mode using PORTO you can set it from the theme settings, check this detailed article first: https://www.portotheme.com/forums/topic/how-to-set-maintenance-mode-on-the-site-using-porto-wordpress/
Regarding the header template condition set to all, You can remove the condition, and then set other conditions depending on the requirements. Check this article to understand better about the conditions:
https://www.portotheme.com/wordpress/porto/documentation/display-condition/
Best Regards,
PThemes Team.