FTP is an old protocol, browsers have removed support, because it's not really modern infrastructure compatible, as it uses multiple dynamic UDP ports and not plain TLS connections. So it doesn't really work well with reverse proxies like Traefik.
You could try to setup Traefik to work with plain TCP for the control connection, but then you would need to use a different port for every target service. Someone else tried to make additional ports available (post).
May I recommend to use WebDAV instead, which uses http?