FTP Server behind router

Hi I have read all sorts of threads that talk about opening a port range for PASV but I am not sure this applies.

Recently my ISP has blocked port 21 for security issues. Before they did that I had no problems with my FTP and my port forwarding worked fine allowing external 21 to come in to my internal port 21.

However now that they have blocked it, people cant connect. I have changed the FTP port to 26 and that works as they can login fine. But they cant seem to list the directory.

Now I know this seems like a PASV problem, and if so, why do I only have this problem now that port 21 is blocked, and what can I do to fix the problem.

When I connect directly to the internet without the router then it all works fine so I know it must be a port issue.

Thanks in advance.