I have this problem also since I updated to iOS 11.2 and none of the suggestions posted on this and other forums have worked. I can sometimes get a purchased ringtone to work, but it is sporadic. I purchased all my ringtones through iTunes. The ringtones are on my phone and I can play them, but when a contact calls me, the default rings. I don't like the default that Apple selected, "Opening" and tried many times to change it without any luck. I was finally able to change it to another ringtone in their list. I thought maybe I was having this problem because I still have the 5S, but my daughter just got her X and is having this issue as well. When I shut down the phone, all ringtones for my contacts will revert to the default. Very frustrating!!
I know it is important to update the software because of security issues, but every time I do, I get a new problem. Almost makes you want to ignore the updates, but I can't take that risk.
Does Apple monitor these discussions on their website? If they do, you would think they would reply and let us know they acknowledge the problem and are working on a fix. There are thousands of dissatisfied iPhone users with this problem as evidenced by all the forums and comments posted which date back several months. How long does it take to acknowledge a problem and fix it! I did submit feedback on the support website, but don't know if they read them.
My husband and I have the iPhone5S and was going to purchase a new phone this year, either the 8 or X. However until Apple fixes the issue of the ringtones, I will not be purchasing new iPhones. Maybe I will look into the Samsung Galaxy.