I’ve been battling a problem with the new Plesk server I purchased with 1&1 IONOS. Everything in Linux and Plesk looked correct but I could not connect via FTP/FTPS as port 21 remained blocked no matter what I did. It turned out the problem was with the 1&1 IONOS dashboard.
Once you login, select “Servers & Cloud” and then click “Select” on your server. This will take you to the server settings panel. Now select Network->Firewall Policies. At the bottom of the list of ports click the “Add Predefined Values” dropdown and select “FTP Plesk” which will add both the active and passive ports. The final step is to click the green tick icon next to the new rows it added and then wait for the server status to show it has finished setting it up.
That’s it! Because this is a new server it has the latest Plesk which already has both passive and active ports setup so once the IONOS dashboard was sorted out then everything worked. You can check this guide if you need to setup your Plesk firewall.