Ensolite is glued to the fibreglass with contact cement. When it separates, it is necessary to clean off the old glue from both the fibreglass and the Ensolite. To do this it is necessary to free up a flap, so you can get under the Ensolite. This may require shaving the Ensolite away from the fibreglass to expose the underside.
After the glue is cleaned off, paint
more contact cement on both the fibreglass and the Ensolite. A low VOC contact cement is recommended. Wait for it to dry. When the two glue covered surfaces touch, they wil bond. You only get one shot at this. Connect the most distant point first. Then connect the edges. Work your way back to close the rest of the gap.
Ensolite is somewhat stretchy, even more so when it is warm. It might be best to do this on a warm day. Maybe have a blow dryer to heat the Ensolite.