The files have to be open but not on the original computer. The computer that opens the files is the host and all other must connect remotely.
Access from outside your network controlled by your router, it points to the local IP Address of the computer it was told to do.
If you move the opened filemaker databases to another computer with a different local IP Address, you have to change the router settings.
The other option is to set the old computer to a different IP Address and new computer IP address to the current router setting
And no, you can not (As far as I know) to set the router to spew the request to 2 or more local IP Addresses.