Issue Tracking Mobile App behind NAT Firewall

I have a mobile App which is running on my internal network. So the PIWIK server sees them all from the 1 public IP. For each user I am setting the _id, but still they all appear as 1 visitor.

This is URL I am using in the mobile app. _id is unique for each user,

https://mycompany.piwikpro.com/piwik.php?idsite=11&rec=1&send_image=0&_id=0004000400040004&action_name=CSROCKS/Newsflash&url=http://mobileapp.tap.mycompany.com/CSROCKS/Newsflash

This is the Visitor details UI see

Thanks
Lloyd