Your router would have to have been configured to map an external ip address to the static ip of the raspberry. It would either have to map the entire ip to go to the raspberry (with perhaps only some ports being allowed through), or else it would have to be configured so that if a very particular port was accessed from the outside that the one port was mapped through to the destination ip address.
If the router has been configured in that mode of only mapping to the destination when particular ports are accessed, then it is very likely that the router permits you to specify which port on the destination is to get the packet. There are some cheap routers that only permit entire ip addresses to be mapped, but any router that permits specific ports to be mapped should also permit you to specify the destination port ("Port Address Translation")
Oddly enough, consumer routers seldom permit entire ip addresses through without supporting port address translation, as consumer routers seldom expect to serve multiple public ip addresses. The devices that support DMZ but not PAT are typically 15-ish years old. They exist.
