[J1mmy]

This can be a headache.

Your router needs to be able to accept incoming connections on port 21, you might also need to specific which computer on the network this rule applies to.

The problem starts with the IP addresses. If you know the correct IP address, everything should work just fine, but as you have 3rd party software managing the IP address you won't always get a connection.

As for SerU, that's already configured on port 21, and I think it also can determine teh IP address, there is little configuration to be done there. You just need to concentrate on the dynamic IP issue.