So I built a dually ranger I used semi multipurpose panel adhesive to mount the brackets to the fenders and this stuff don't seem to hold like it should any ideas for a better adhesive thanks in advance

 

I doubt any glue will hold dual fenders on, to much wind load and not enough surface area between fender and body, or bracket and fender to hold it.

They will need to be bolted/screwed on

What kit did you get?

Brackets are usually there to support fender not hold it, fender should be screwed to body panel

 

I have pretty decent brackets made that are two inch wide bolted to the bed but I'd like to not have screws or bolts through the fenders as I'm trying to keep them looking like they did on the 350 they came from