Up until last fall when we built a steel roof pergola over deck I had 2 9ft market umbrella at each end of pool deck. For each umbrella I mounted a section of PVC pipe , slightly larger than the umbrella post, up against the deck railing on one of the 4 x 4 posts,, resting on deck floor and up to top rail.
Inserted umbrella and drilled a hole through the PVC and umbrella post, and put a hitch pin through both and locked in place, same clip you use on a trailer hitch ball to keep lock lever down.
Worked great and we get wind all the time. Depending on your railing, you may have to notch out the upper hand rail to conform fit to PVC and lets it sit straight and not at an angle. Not sure if its a metal poste umbrella as ours were wood. If I did not put a pin through the hole on windy days they would launch out of the tubes and be instant umbrella lawn darts. I usually slid them out and stored them before extreme high winds, such as over 40 mph as I was afraid they might snap over even closed and not to mention fade from sun, red will be pink in no time