Your spoiler will be working.
When you activate the spoiler manually using the button, it raises to its highest level (normally over 98 mph for automatic activation). At this height it can be seen through the rear view mirror from the drivers seat.
When it activates at 55mph automatically, it only rises to its lowest position which cannot be seen through the rear view mirror.
Either drive past someone at over 55mph and get them to tell you if it was raised, get someone to follow you in another car, or,
tear some paper into small pieces (confetti size) raise the spoiler using the button while parked, put the paper under the spoiler (when it’s dry) then lower the spoiler with the button. Take a drive over 55 mph park up, raise the spoiler & see if the paper has gone. If so, it’s working.