The confusion came from the different meaning we used for the same terms: "box" in my definition is the STB while for some others this is the modem/router used to access the internet.
iptables is usually only used at router level, the STB being just a LAN device in your network like any other. Hence the OP's expectation that accessing his STB via internet should work with port fwd defined at his router level ... and without any iptable defined on his NAS, for example ...
Edited by gspock, 12 February 2021 - 15:58.