First things first, head over to your SFTP tab and scroll down to find the option:

  Leave uploaded files intact. We won't change any game files unless we're installing a new version of the game. This will let you upload mods through SFTP.

With this enabled, go ahead and click "Save Settings"

Use a SFTP client to connect to your server. We recommend the use of FileZilla make sure you download the one for CLIENT not Server, otherwise you will be very confused later. After you have installed FileZilla or your program of choice. Open it up and connect with the details found on your SFTP page. You can use the Quickconnect in the bar.

Now that you are connected, navigate over to the /7daysserver folder. In here you will create a new folder with the word Mods as you see here. (Note: If you have never installed mods/modlets on your own client seek a guide on how to before attempting an install to a server.) Now that you have a Mods folder, this is where you will be able to place all Mods so long as they work on your client they should work here too. If not reach out, and we can assist.