Hi! Thanks for the question and I have three ideas. First, as @DanMorris said, get a Physical Therapist involved and find one that has experience with cyclists. They will be able to help pinpoint what is keeping your glutes dormant. The second thing I would recommend would be to check out @Jonathan post. He has many great exercises for opening up the glutes.
The third thing is a simple one and worked for me (so N=1). I start by laying on my back, with my hands underneath my buttocks, then gently contract each side of my glutes about a dozen times. After that, I move to a chair and repeat the same process. Finally, I’ll get on the bike. My glutes only start firing once I get to around 75% of my threshold and I’ll press my finger onto my glute to make sure it’s active. Bike position/fit and how hard you are working will definitely affect the glutes.
Hope this helps!
-Jesse