In principle: An architecture is recommended in which the device in which the SIM card is installed establishes a connection to a server and transmits data there. Making devices accessible via the free Internet involves security risks such as botnets and enables attacks.
Unfortunately we are not able to assign public IP's for either Digital Republic Flat and Digital Republic Flex at the moment. You provide private, dynamic IPv4 addresses.
DynDNS: Technically possible, but not recommended for security reasons. Unfortunately, we are not allowed to offer technical support and advice here.
VPN (Device to Server): A VPN connection can be set up with some router manufacturers. The security is appropriate with correct setup. Since the setup is strongly dependent on device manufacturer and software, we unfortunately cannot offer any technical support.
With the port scanner, you can check if this forwarding is working correctly: http://www.dnstools.ch