Looking at my embedded Linux device. Connection refused to default ports with either ssh or telnet clinet, but not the ftp port. I have root pwd because the web front end of the device uses the true root password for administrative web page access. So am ftp'd in as root. Is it possible to configure and activate telnet or ssh with only ftp access so that I can remote in to a command line on this device? (In the mean time I'm going to poke around inside the case and see whether it has a serial tty console.)
It does not matter whether Linux is embedded or not. It doesn't even matter whether it is Linux or not. FTP does not let you run commands on the remote system.
You would also need to login to FTP as root to able to access the system config files, and hopefully that is disabled for appropriate security. Really, FTP should be disabled for everybody since the protocol is inherently insecure.
You say you have the root password so you should have full access to the device. Without details of which device you're using it is hard to say more about how to make this work. Somewhere in the web interface should be what you're looking for. Try searching through the docs or screens for "service", "init", or "systemd".
Another possibility is that if you can plug in a USB cable to your device you could get a terminal session going that way. https://learn.sparkfun.com/tutorials/terminal-basics/all might help you get this going.