On both iOS and macOS you can hold a key on the keyboard to get the accent menu from which you can convert an e to ë by pressing the indicated number next. On my Mac this feature disabled itself after one of the recent updates. Pretty annoying and I tried a lot of things to get it back. Including reboots and changing the keyboard languages. Nothing helped untill I came across this bright comment. A simple tweak to re-enable the press-and-hold feature without rebooting or messing up other settings!
How to fix
Open Terminal app from Spotlight search or Launchpad. Then copy paste the following line and hit [enter] key:
defaults write -g ApplePressAndHoldEnabled -bool true
Now logout from the Apple menu, log back in and it’s resolved!