Barrier free shower lip adhesive

I have a friend who is confined to a wheelchair, but lives independently. During the renovation to make his condo appropriate for his lifestyle, they installed a barrier free shower with a collapsible lip, so his chair could roll right over it and it springs back to height.
The original adhesive between tile and the dam failed, and I used aquarium sealant to keep in on, for about 2 years - but it's gone again.
I've inquired from my epoxy dealer and he suggested West G/Flex, although not knowing the material makeup of the lip, he wasn't sure of the adhesive bond-ability.
Anyone faced this issue, and how did you make a "permanent" repair?

If you truly want it permanent-- 3M 5200 marine adhesive.

And if you want a "little less permanent," 3M 4200 marine adhesive. 5200 is very permanent indeed! If the house if ever sold, or such, it may be important to reword the shower to have a more substantial threshold structure, and removal of the adhesive may be important for that.

Either is a good choice and will do the job.