Client Expects Port 21 But FTP-servers Have Same IP Address?

My setup is as follows: as a part of a global network, I have two separate measurement stations collecting data in the field. They are working with some kind of embedded linux system and is running an FTP-server so that I can download collected data. I have some control over the stations, which e.g. means that I can select the port used for the FTP-server (default is 21).Unfortunately, for this particular set of measurement stations, they are connecting via FTP-over-radio-link to another station which is connected to my computer via a router (on the server-side) and I can only "see" their common external IP address. If I have understood this correctly, they can not use the same port for FTP traffic, right? The router used is some (for me) unknown brand with unknown capabilities, so I don't expect/want a solution at that end.Now, to my problem: as a part of this global network there is a custom software running on my computer that automatically connects to all the different stations to download and process the data. This software runs on any recent version of Windows, but I'm running Windows XP right now. There is no way for me to modify this software at this point (for the future, maybe, but not now), and unfortunately it has a severe limitation: it MUST use port 21 for FTP-connections.My first attempt was to set the FTP-port to 2121 for one of the servers and create another IP address on my client network adapter and try to forward port 21 of this new IP to port 2121 on the server/router side, but due to lack of knowledge and experience I did not manage to get this working fully.