Running a Bitcoin node (behind Firewall / NAT or anything else? disallowing inbound connections)
You may wish to attempt the following: Your router most likely offers you a GUI so you can adjust a few settings. Use your browser to connect to your router whose ip address is something like 192.168.0.1. This will bring the GUI to your browser (you will need to find out the login details giving you access). Once you are connected to your router, there is probably a section called 'advanced settings'. Within that section, you may see a subsection called 'DHCP reservation' which should allow you to choose a fixed internal ip address for your raspberry. Once you have done that, go back to advanced settings and find a subsection called 'port forwarding`. This will allow you to instruct your router to forward any connection attempt on port 8333 to your fixed ip address.

In fact, I have just been paraphrasing bitcoin.org