I solved the notifications not working on my Galaxy S8. No matter what I tried I could not get the text notifications to work on my phone. All notifications were allowed at every place in the phone possible. I have tried every fix posted on the Samsung forums and on the web and none of them worked. I did not do a factory reset I read that doesn't always fix the issue. Yesterday, I downloaded Textra to test it and still the notifications didn't work. I downloaded the Chomp Text app and my notifications are working again properly. Solved at least for me. Best of Luck.