I used a function to make my mobile faster in polish version. I don’t know how it called in english. I choose a aplications to clean and make free my RAM memory. After that I can’t open any website, and I have a statement: This website is unreachable, failed to found DNS server address.
I have Android 6.0.1. Phone: Samsung J5, SM-J500FN
https://forum.avast.com/index.php?board=71.0 (English)
or
https://forum.avast.com/index.php?board=50.0 (Polish)