Here is another quote:

"Just a note to add that if you use the version of those Sunon fans with the "F" 3rd wire output (frequency or "tacho" output) then the switch will register a fan fail - you need the version that has the "R" (Rotation) type 3rd wire output which is a basic constant 5v or 0v output depending on whether the fan is stalled or running"

https://www.youtube.com/watch?v=1_dM0pBRfOI&t=0s