The photo does not show. However, there are metal support brackets which are L-shaped. They screw into the planter, then have holes for screwing into a wall. Maybe you can nut and bolt or use some heavy hardware clip (stainless steel) to attach the top hole of the bracket to your iron railing. Sorry, but the photo does not show, so I'm randomly guessing how this would work.

Thanks! Yes. I think I'll try to hang it from the bar in the picture (photographed from above).

Maybe I can find a way to attach it to this top bracket (like the one I drew below).

Thank you!!

This is a great project! There are a million ways to do it: brackets, clamps, clips, beams, wires, ropes, magnets, etc. I would look for the simplest, cheapest, and least conspicuous solution that will hold up securely when the planters are full of wet soil.