I can't remotely manage and configure the RouteFinder from my office PC . Also I'm not able to telnet to the RouteFinder using the global IP.

It says device not found.
Make sure you have version 7.29/4.58 or newer fw/sw for the RF500S.
Go to http://www.multitech.com/en_US/SUPPORT/Families/RouteFinder/firmware
and download the latest version of the firmware and software.

Please read the README that you downloaded, it has instructions on the upgrade process.
After upgrading to the latest firmware, then enable remote webadmin,
this will allow you to remotely manage and configure the routefinder.