In order to activate and save the Nat Loopback function in the device, please follow the belowing command.
 
1) Telnet to the device, key in the password
2) go to SMT>24 System Maintenance> 8 Command Interpreter Mode
3) type "sys edit autoexec.net"
4) type "i" (insert command) then follows by "ip nat loopback on"
5) press "x" save and exit
6) type "sys view autoexec.net"
7) there will be one new command line on top "ip nat loopback on"