I am sorry for people experiencing this, because it happened to me, too, a while back and I was very sad. I recall another thread with the same problem (and no solution). It is because most phones in use nowadays have nano size SIM and most of us don't use the N9 100% of the time, and we do SIM swaps and use the dangerous adapter.

When I pulled out the nano SIM adapter, it bent the pin or pins (very hard to see). But then I did not use the phone for a while, and apparently the pins slowly moved back, because after a year or two I tried it again, and it worked again. I lucked out. I had even searched on eBay and found a replacement part and was going to figure out how to do the soldering, and I am really glad I did not have to do that.

Maybe try to move the pins back in place with a needle or something? I probably tried that, but it was a long time ago now so I'm not sure.

June is the final month of 3G network in the U.S.A. (through T-Mobile) and I am using my N9 as my main device 'til the very end. (In fact I have not swapped out my SIM into another phone for many months.) So maybe that is a bit of consolation, that you will probably have to settle for non-phone uses of the N9 soon anyway, because 3G is being swapped for expanding 4G and 5G networks.

I will keep using the N9 after its telephone days are over. I have a super cool FM Radio tuner app on it (many phones do not have FM radio) plus other very cool things, so I won't totally stop using it. I even have a spare battery. And spare screws, and the proper screwdriver (very important, which I also learned the hard way). So much pain and struggle (but also some fun) to keep this lovely phone going over the years.