[Sholander]

How many characters did you have to add? My Firestick 4K has 64 characters at the end that are missing; do I have to write them ALL at the end?

Adding all of the characters through the Netflix addon ESN editor worked for me, too. I found that the full ESN was stored in the settings.xml file, so for my shield I used a mobile phone with the Android X-plore app to open the xml file and copy the string and the Kore app to paste it into the addon ESN editor. That saved me the aggravation of trying to enter it with a remote.

This does not help on my Firestick 4K, UHD/HD just does not work. I got the full ESN (48 + 64 characters) from official Netflix app and had to enter them manually with my WiFi keyboard.
I had to put "only" the last 64 characters at the end of existing ESN string, double checked for any error, clicked on "Apply changes now". It prompted me to now try a video; but again it played only in 540p. When I looked again to check ESN setting, the string has returned to former default - "Official Firestick Netflix app" string without last 64 characters that I just put in. 
It looks as if Netflix addon v.1.20.2 on my FS4K does not store the additional 64 characters. Don't know if this could be related to my Kodi version - Maven's Kodi 20 Nexus beta1 for Firestick - or to something else...

EDIT: figured it out finally; to keep additional 64 characters written to the end of existing ESN string, after I wrote them in I had to press "OK" (NOT Apply changes now). After pressing OK I restarted Kodi and checked again ESN string. Now it shows all values, complete with additional 64 characters; and UHD Dolby Vision with Dolby Atmos are now playing OK, as before.