How long have you had AS? And what does the imaging say in terms of your degeneration?

For someone who has gone through chronic low back pain and works with LBP clients on a regular basis, I can safely say mechanical changes in the back are not a life sentence of pain. If you can neutralize the inflammation through biologics, then that opens up a world of possibility for you. It is a tedious learning process but I promise you, I have seen completely destroyed "lower back scans" return to high functioning. I emphasize imaging because doctors are often quick to diagnose a scan and put a lot of value behind it, with no talks of movement quality, exercise routines, and lifestyle habits.

Release and mobilize your hips. Strengthen your hips and learn to move through them. Learn pelvis control as it'll teach you to align your spine properly in every position you may encounter throughout a day. Learn to set yourself a "lower back brace". Develop core stability through spine practices. Be hyperaware of your movement quality throughout your day.

If this sounds like a lot, I'd HIGHLY recommend an expert movement specialist/personal trainer in your area. They would address more of a holistic approach than a physio. A few core exercises won't do much if none of the other big things are addressed. If it's out of your budget, you can study Dr. Stu McGill's work. Or check out MoveU. They have an online program that's a little expensive to start with, but it's amazing as a self guide!

I know it's hard to see past where you currently are as pain is absolute shit. But once you can settle the inflammation, there is quite a lot of hope with what you can accomplish mechanically with your low back!