InstantVNC is a zero-config, zero-install application that allows remote access to the Windows PC, it requires no configuration or installation and allows remote access to the Windows workstation where the applciation runs on.
Simply distribute this binary to your target support users, and youll be able to remote support them regardless of any intervening firewalls or routers.

The license of this software is Free, you can free download and free use this remote access software.