Websoket with API Gateway, ECS and VPC link

Hi, I have a WebSocket deployed in ECS with load balancers on top of that and use the VPC link to link the API gateway with the ECS. For some unknown reason, I was able to connect to the API Gateway WebSocket successfully but can't send the data to the ECS. I send the message with the action "sub" but nothing hit the Docker container in the ECS. Is that because it is not possible to have the API Gateway WebSocket with load balancer and ECS connected by the VPC link? Can that be achievable? Or is the API Gateway WebSocket can only talk with Lambda?