Form validation plugin result in 404
Sounds like you need to allow anonymous access to your controller action.

I guess it works on your computer because you are logged in and thus have a valid session, but then when you try on mobile, you haven't got a valid session, your controller isn't set up to allow logged out users to post to it and therefore you get redirected to /login

To allow non logged in users access to this action, its simple. Add either one of these just at the top within your controller class.

// Will allow anonymous access to all actions within this controller
protected $allowAnonymous = true;

// Only allow access to specific actions
protected $allowAnonymous = array('action addParticipant');