Hi there and welcome
Not it is not via the IMSI, but it can be via www.icloud.com
When you go to www.icloud.com and sign in using the same Apple ID as you used on the iPad.
Are you there able to locate your iPad?
Please let me know.
ok... I only see here "calling AT&T, Find my Device on ICloud, police"
But the iCloud option is the only option to track it sorry.
Please do not shoot the me on this, I can only tell you the reality of the situation, and if you detached it then it will not be possible to locate.
Only AT&T can do this
No one outside of AT&T has access to their infrastructure to do any searching