I would probably make a struct of your enum and bool value, then make an array of those to iterate through more easily.
Otherwise I think you can do something like this;
Basically increment an int, while flipping through your enum, to find your start point (since enum index isn’t set). Then flip through our unlocked bool array based on that starting point, until we find the next unlocked one, and then set our weapon enum to the new value.
This only does the increment, you should be able to modify it easily enough to decrement as well.