The Control tab appears only when the device is online.
You have to enable the VPN in WE500 to see it as online in the Portal. Go to Administration -> Networking -> VPN and select the Enable** checkbox.
After you click the Save button, WE500 can take several minutes to register in the portal and become online.
However, you have first to configure the GPRS/HSPA connection otherwise WE500 cannot reach the Portal.
The serial number that you have to enter is the number that you see in the web interface in Administration -> System -> Info -> Hardware serial number
Hint
Complete manual available at https://nethix.co/doc/en/portal/portal_user_manual.html