Unaccomodating workplace

How do those of you who work 9-5 desk jobs stay comfortable? Especially if you have SPD or pelvic/lower back pain. I was told by my boss today that I can’t prop my feet up or lean to the side in my chair as it’s “unprofessional”. I physically cannot sit straight up with both feet on the floor in a desk chair for 8 hours a day. I have since gotten a doctors note that I plan on bringing in stating that they need to accommodate me but I’m not sure how everyone else is getting by. Thanks in advance!!