Fixed my issue. I had to change my FTP server settings. I'm using Xlight FTP server and had something enabled that prevented connection hammering. Disabled that and I'm good to go.
I notice my connection breaks trying to upload a large file (18 GB). The connection breaks every 1 GB. I was able to get the resume working, but the option reconnecttime doesn't seem to work. I've tried many variations, but here's the one I thought would work:
"C:\Program Files (x86)\WinSCP\WinSCP.exe" /command "option reconnecttime 1" "open sftp://username:password@server/folder" "put ""C:\largefile.zip"" -resume" "exit"
I can get it to resume by manually executing that command again when the connection breaks, but I need it to automatically reconnect and resume.
What am I doing wrong?