I had in IR sauna for about 4 yrs. I now live in an apartment building that has a steam room and electric element sauna. When I had AS, it gave temporary relief. However, I found exercising and getting blood to the muscles provided even more relief (even though it was painful to do sometimes).
Note: Many people with AS end up with fused backs as they get older (such as my father). So from a young age I knew I had to maintain flexibility and preserve the gap between the vertebrae. To do this, I exercised & stretched regularly (despite how painful it was), drank lots of water (with sea salt) and made regular use of an inversion (traction) table. So when I finally figured out how to cure the condition, my back was well preserved & I had no lasting damage.
IR saunas work much better as the far IR rays travel deeper into the body (up to 1.5") causing you to sweat/detox from deep within the muscle. Other saunas require much higher external temperatures but only heat you up on the surface and are therefore less bearable & effective. Detoxing is the first step to healing your body. Detoxing through diet is more effective than saunas, but if you have the funds and room, they are certainly better to have one than not. IR rays have also been shown to detox EMF's (as do zeolites). The only problem is that parts of the IR sauna emit high EMF's also. There are some more expensive IR saunas with shielding for EMF's.
Also going from hot to cold temperatures strengthens your immune system. I like to have a hot sauna then have a cold shower, then back in the sauna. It is feels a bit extreme at the time, but is quite refreshing afterwards.
If you can't afford a sauna and would like something to help warm up the muscles to provide temporary relief, you can also buy IR back pads/belts. I had one that covered my whole back and I used it at nights or in the mornings in bed.
Now that I healed my Leaky Gut, I don't get back pain anymore, so I sold a number of the relief items I had. I did keep the inversion table though, I think they are great.