Ive used a polymer adhesive for mine, but Sikaflex 291 (marine sealer/adhesive) or Sikaflex 227 (auto sealer/adhesive) is commonly used. Both the Sika's are polyurethane not silicone. Screw the panel to the brackets, and glue and screw the brackets to the roof and proper surface prep is vital. If the panel is loose I suspect swapping the steel for alum angle would be a better long-term solution. Both Sika 291 and 227 adhere to alum and fibreglass well, but in all cases testing your application first is always advisable.
If one side has detached, and the adhesive has lifted off one surface that indicates wrong adhesive or poor preparation. Either would suggest the other side wont be far behind it.