Hello,
Can you detail what exactly you want to accomplish? If I understand correctly you want to access your device via public IP? First of all, your ISP has to be provided a public IP for your SIM card. If you have a public IP, then navigate to the System > Access control and enable Remote HTTP/HTTPS access, save and apply changes. After changes were made you will be available to access WebUI by public IP address.
Here is the link to the detailed instruction: https://wiki.teltonika-networks.com/view/RUT240_Administration#Access_Control
Regards,
Klaidas