The external ip is the ip assigned to you by your isp. There are several pages on the internet that will give you your external ip.
If you are behind NAT you may not be able to FXP as in most cases NAT and FXP are not compatible.
At any rate the problem seems to be with the service, you best bet would be to contact them and see if incoming connections are allowed and active mode (non-passive) connections.
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