You can download the Modpack installer from their site here (just tell it to use 1.11.0 so it matches your client version): Feed The Beast - FTB Presents Direwolf20 1.19
Run the installer locally in a folder on your PC, then use AMP’s built-in SFTP or web File Manager to upload the files
Or alternatively, you could downgrade your client version since the server is already up and running