What can I attach stone to my stairs with?

[tensiden]

The stones have started coming loose on our exterior stairs. How can I fix this in the best way?

As an emergency solution, there's PU stone adhesive, but preferably I would use some form of mortar if it's available and possible.

Do I need to fix the interior? I'm unclear on the construction, but it would be nice not to have to tear down the entire stairs and start over.

 

[useless]

Biltema's stone glue. Tell yourself it's a temporary solution.

 

[tensiden]

Biltema's stone adhesive. Convince yourself that it is a temporary solution.

Temporary solution is perfectly fine. But would prefer to avoid polyurethane and other similar materials if possible.