After upgrading my phone to Android Lollipop, mobile tethering stopped working. I rang my operator and they said my account allowed it so it should work fine. They then ran me through a bunch of steps and it still didn’t work.
I found a long discussion on XDA developer that I was going to review eventually and then my colleague sent me through this fix to type into an adb shell:
settings put global tether_dun_required 0
And now it works. Hopefully it will help you too.
Thank you to silvrback and you can see his more in-depth explanation here: https://pmf.silvrback.com/fixing-tethering-on-android-kitkat