This guy talks about getting it to work using a fake transform:

https://stackoverflow.com/questions/52420672/ros-rviz-how-to-visualize-a-point-cloud-that-doesnt-have-a-fixed-frame-transfo

This thread contains some info about what a transform is:

https://answers.ros.org/question/328839/

Somewhat useful troubleshooter:

https://www.youtube.com/watch?v=b9YZITmCWe4

Excellent, full-featured explanation:

https://www.youtube.com/watch?v=QyvHhY4Y_Y8