Most CompleteFTP installations start off in trial mode. If an installation is not activated before the trial ends then CompleteFTP will automatically switch to developer mode. In developer mode incoming connections are accepted only if they're from the local machine.
The method of activation of CompleteFTP differs between the Free Edition and the three paid editions, Standard, Professional and Enterprise.
If you are trying to activate a very old version of CompleteFTP (pre 9.1.0) that does not support automatic activation, then see here.