I had never heard of AS but for what it's worth I think that we are all facing a host of 21st century autoimmune diseases that may be caused (in part) by our diet and environmental factors.
Although my situation in no way compares to yours, I think I may have a condition called leaky gut after a lifetime of eating legumes, nightshades and specifically proteins like gluten which are part of the lectin family (or something along those lines - I'm not a doctor). Also beer, lots of beer, which certainly didn't help. It may be exacerbated by molecules in genetically modified foods that normally don't appear in our diet, and by pesticides/herbicides like glyphosate that wreak havoc on our gut flora. These are present in large quantities in processed foods high in soy and corn, especially fast food. IMHO this is why so many young people are getting type II diabetes and obesity.
I just did a quick search for the overlap between our conditions and stumbled onto this page:
http://stopautoimmunity.com/en/2016/11/20/ankylosing-spondyl...
I highly recommend glutamine. It repairs the single-cell lining of the intestine to prevent microbes from crossing over into the body cavity. In my case I've been suffering from acne for the last couple of years (I'm 41), severe fatigue and joint pain from heavy gym sessions. I've only taken glutamine a couple of days and my acne has begun clearing up.
I'm wondering if autoimmune diseases (like arthritis) might be caused in part by our immune systems constantly fighting microbes anywhere our cells have contact with the outside world, so for example mucus membranes, our digestive tracts and our joints. Any supplement that strengthens those like glucosamine/chondroitin MIGHT have a therapeutic effect on these conditions.
Also CBD oil can work for inflammation, but you might want to wait until you're older to try it as the last thing you need at your age is dependency on a mood-altering substance. Also there has been little literature about the longterm benefits or side effects of it as it has been illegal in the US for so long. I really view it as a remedy for older folks.
If it was me, I would try holistic/lifestyle changes such as avoiding foods that cause inflammation, staying away from opioids, strengthening bone and connective tissue (see people like Mike O'Hearn's videos on YouTube), etc etc before trying an experimental treatment like stem cell therapy. Just remember that western medicine can be out of touch when it comes to complex systems and holistic effects. But, I am optimistic that great progress within these large search spaces will happen in the next 5-10 years from advances in AI and having big data to train on, if you can maintain your health until then. It sounds like you are on the right path with your diet. Best of luck to you!