Forward outbound internet from TG582n router to 3com router

[#1]

Hi All,
I am having problems with my technicolor TG582n port forwarding.  I want to setup a ftp server in my home network and connect to it remotely but using a different port number from outside - lets say port number 59987.  I set up the router to port forward anything external from this port to port 21 of my internal address 192.168.1.4.  I've set it up for UDP and TCP.  I've not set any trigger settings.
The problem is I never seem to be able to get passed my router when I use any other port other than port 21..  As soon as I change the incoming port forward back to 21 it then works fine.  I have tested it with the plusnet firewall completely off too.  Having the firewall off and ftp set to port 21 is a security issue and so I need to be able to use a different port.  I've followed various instructions and so am pretty sure I'm setting it up correctly.
Anyhow, as this was proving more difficult than I thought it should be and concluded that perhaps there is a bug in the router's firmware I thought I'd be able to simply test this out by using my old router (3com) as the gateway and simply plug the technicolor into that as I know for sure that the 3com router will port forward correctly.
However, I hit across a problem and I don't know how to tell the PCs connected that the gateway to use is not the gateway that the technicolor is pumping out (because I have kept it's DHCP on to save me having to setup everything on the old router) but to use the gateway of the old router.
In more technical (!) terms, how do I keep using the DHCP from the technicolor router whilst routing all internet outbound stuff from the technicolor gateway (192.168.1.254) to my alternate gateway (192.168.1.20).
More info: The 3com router is a fixed IP address and it's dhcp is off, the netmask are both set to 255.255.255.0
Can anyone advise how you can do this?
Many thanks inadvance
Rod

[#2]

Did you open up a range of ports for the DATA transfer and then tell your ftp server to specify those ports to the client?
I suspect it works on Port 21 because there is probably an FTP ALG (Nat Helper) setup in the Router that's monitoring the un-encrypted FTP control channel and dynamically forwarding the data ports on the fly to allow the actual communication to take place.
But I very much doubt the router checks any traffic on others ports for the presence of FTP.
Usually if you've forgotten the Data ports you'll connect, login but as soon as your client gets as far as trying to list the directory it will just sit there for a while then time out.
By the Way do you actually NEED ftp open or could it be replaced with something like SFTP? (FTP over SSH) which is all nicely encrypted and works over a single port.

[#3]

Hi, thanks for the reply. I was trying to get it to work on simple ftp at first and then possibly setup sftp later.  I'm probably a little rusty on setting up ftp ports so perhaps that's the problem although I didn't even get as far as being able to login.  I'll perhaps repeat the tests and post up the logs from the router.
To confirm,  for ftp you need port 21 (initial link up) and then port 20 for data.  I didn't do the port 20 bit so will give that a go.
In the meantime (and perhaps I'm thinking that I should still test out my old router)  could anyone help with the other issue?  After doing some reading around,  do I simply need to change my 3com router so that it's in bridging mode?  I'll have to see if it has that. It's a 3com ADSL 4 port wirelessmodem router (OfficeConnect 3CRWE754G72-B), quite old but still works.
Thanks
Rod

[#4]

Hi All,
Any update on my query about turning the 3com router into bridging mode so that I can give the port forwarding a test on this device?
Many thanks in advance
Rod