Hi Stefan,
How have you enabled your Portal access over Internet?
Are you using Web Dispatcher or any reverse proxy to access the Portal from outside network?
In case you're using SAP Web Dispatcher then to access WD ABAP based functionality through Portal over Internet, you need to define and open a Port that should be accessible over Internet and that it will route all your back end server requests to ECC system.
You can also open & expose the port of your ECC system over Internet to access the WD ABAP functionality but that is not recommended due to security constraints.
In case, you do not have any security mechanism so far in your landscape to access the ESS / MSS functionality over web, please explore the SAP Web Dispatcher / reverse proxy options.
BR,
Anurag