You said "restarting" but have you tried "restoring"? (i.e. backing up, erasing, reinstall iOS, reinstall backup) Sometimes an iOS update can go sideways and you have to restore the iPhone:
After this update, I suddenly don’t get any sound out of the right earbud. It isn’t the earbuds-I tried another pair and same issue. I checked and the sound is balance and not set for monk. These are regular headphones and not the bluetooth ones. I’ve tried restarting and cleaning the port.
User profile for user: amandafromgreenfield
Question: Headphone issues after 11.2.6 update