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I am trying to set up an FTP server on a machine on my network. When I go in to the router settings and then to IPv4 Port Mapping, I can click add to open a dialogue box to "Add Port Mapping". I can put in all the details until it asks me about "Public Port" and "Local Port". When I type, nothing appears in those boxes, so I can't complete the entry.
I have no problems entering data into those fields, so it should definitely work for you.
I suggest you try a factory reset on the router, but first make a note of the firmware level, then remove the DSL cable from the router and select Settings - Configuration - Factory Settings. Once it had rebooted plug the DSL cable back in.
In my experience that is the only way to get a new firmware, if one is due. (the firmware update checker simply doesn't work)
Let us know if it works.