If all_ports_natted means that whatever comes to the ISP’s router public-facing IP address:port X is forwarded to a configured private side address:port X, and you’ve configured that private side address to be the one of your Mikrotik, I’d assume that the reason why FTP to that public address doesn’t work at all is that you don’t use connection-marks to note down which WAN to use to route out the responses. See this topic for the details (check the last paragraph of that post first).
But there may still be an issue even if/after you have configured this properly, as FTP uses one TCP session for control communication and creates other TCP sessions for data transport. The parameters of the data transport session are communicated using the control session, and the firewall must replace the private side IP and port in the control message by its public side IP and port. The firewall needs to be told which TCP port is used for the control communication so that it would treat the control connection that way (it doesn’t check each and every TCP session to be a FTP control one), so if this is not configurable on the ISP’s modem, you cannot expect it to recognize an FTP connection on port 2121.