in your shoes: i would switch to different connection method to see if it’s curl that’s causing your problem. ssh w sshfs would be the easiest switch since it doesn’t required you to modify the ftp server, other than to install the ssh server.
if it’s connects, then you know that curl is your culprit and you can further narrow down the problem; or just stick w ssh.
if it doesn’t connect, then you know that curl is probably okay and that your problem is elsewhere.
i bet that they work the same way that the cd to cassette adapters worked except, instead of a stereo aux jack with continuous audio signal, it uses an digital to analog converter and the rest of it works the same way.
here’s video from on of my favorite youtubers who does a really great job at explaining how consumer electronics work and he explains how the cd to casette & bluetooth adapters work: https://www.youtube.com/watch?v=dH4n8fUjtLQ