You may need to use Passive FTP. (For, when using Active FTP, your computer needs to open up ports to accept incoming traffic as well, which may be blocked by the router/modem. Not all servers like clients that use Passive FTP though.)
EDIT: Even using Passive FTP may yield problems because routers often rely on NAT, which changes ports on the fly. However, the FTP protocol uses PORT and PASV commands, which tells the other side which ports are being used. Here, the router messes up when changing the ports. So, a good router must know that it is handling FTP and actually change the PORT and PASV commands to make up for the port changes.
A DSL modem often has a router built-in, though it may not have a switch (or, a hub in the old days) in the same housing as well. So, a DSL modem often supports multiple computers, but needs an additional switch to actually allow for connecting multiple wired computers. Just counting the ethernet ports does not tell one if a device is a router or not.
According to the user manual, the Siemens SpeedStream 4200-Series has a built-in router. So, you are now using two routers. When using UPnP to allow your software to request to configure port mappings, then FTP may in fact work, but using two routers may mess up things a bit. You may also encounter problems for Instant Messaging (or: video or audio during Instant Messaging), or for certain games. (Note that enabling UPnP may also allow trojan horses (or viruses) to request port mappings, so if not really required one could leave it to "off", or to some safe settings like "discovery only".)
(Like MarkM suggested you may be able to put the first router into "bridge mode". That kind of connects your second router to the internet directly. When using the router-functionality of the first router, then you can set up the second router in "bridge mode" instead. In either case, one router should suffice and avoid problems. It may be a bit hard to get done though. To ensure your friend did not leave some undocumented configurations in the SpeedStream, considering resetting it to factory settings as well.)