That would not be very common - to make your router a media stream server available from the outside.
However, you can do this now:
- Copy the /etc/minidlna.conf file as /etc/minidlna.alt.
- Edit /etc/minidlna.alt to change the interface to whatever you want, and save it.
- Save /etc/minidlna.alt to nvram so it survives the reboots:
nvram setfile2nvram /etc/minidlna.alt
(or use any other means to make this file available again after reboot).
After you restart DLNA server from GUI, it will pick up the new configuration. Of course, in that case you won't be able to change the media server configuration from GUI - it will use whatever path you specified in the alternative config file. But you still will be able initiate a forced rescan from GUI by checking the "Rescan on the next run" box and using the "Restart Now" button.