I set up an RPi3 with Bitcoin full node 13.2. It is running Raspbian the latest version and is up to date.
My router has a static IP on the public internet. My RPi3 is behind NAT.
I configured my router to port forward 8333 to my RPi3.
The RPi3 is pretty much stock, so I don't think there is a firewall running. If there is I don't know about it or how to configure it.
The bitnodes site says my RPi3 is not accepting incoming connections.