Idle timer to keep user logged in

I am wanting to have an idle timer for contao where an ajax request is sent to keep the user active if they are doing stuff on a page.
I'm also wanting to do this from a non-contao subdomain which complicates things.
It shows as logged out when sending the request to keep the user active, thus not keeping them logged in.

I'm using Contao 4.9.

I'm not sure if it is because the cookie is being set for only one subdomain in Contao, would be nice if the Contao cookie was set for all subdomains?

Thanks