Not sure where you got this from, but it does not seem to be a part of Traefik v2, according to the docs.
I would expect that you do not need any Websocket specific configuration in Traefik, it seems to work with just a regular http/https configuration.
WebSocket "is designed to work over HTTP ports 443 and 80 as well as to support HTTP proxies and intermediaries", thus making it compatible with HTTP. (Wikipedia)
In general I recommend to enable Traefik debug logs and access logs, furthermore check the Traefik dashboard to see the current configuration.
It helps if you post your full Traefik static and dynamic configuration, and docker-compose.yml if used.