I too am having this same issue with my S8Plus SMG955U. I did a bit of research and found this page about an issue with the S7 (https://www.snbforums.com/threads/ss-gal-s7-android-phone-tethering-not-working-384-3_beta3-rt-ac3100-with-partial-solution.44599/)
I'm not certain that it's the same...