You need a frost-proof silicone, a building- or better well- silicone. No bathroom type, indoor stuff usually is not up for the job.
You can also use screws to bolt it down, then you need special ones that are watertight, in german we call them a "
Spenglerschraube" (roofing screw? They have a kind of special-formed washer or collar, with a rubber/silicon ring).
Best made of copper, or here stainless steel would be better. Then you have to drill first, otherwise you will have cracks in the fibreglass.
Whatever, you have to have a drip edge or an eaves gutter that keeps the water away from the critical edge. Those guys should at least have installed an overhang.