I have my Raspberry Pi 3 connected to my WLAN with the on-board chip. Now I want to connect my TV's Ethernet cable (it does not have WiFi) to the RPi3 to connect to the Internet from my TV (Ethernet tethering).

Enabling Ethernet tethering with LibreELEC should be very straight forward with connmanctl:

connmanctl enable ethernet connmanctl tether ethernet on 

However the last command fails as Ethernet tethering cannot be enabled.

The problem is that I cannot add ethernet to the list of tethering technologies in /etc/connman/main.conf:

TetheringTechnologies = wifi,ethernet 

This is because the file is read-only.

How can I enable Ethernet tethering then?