So now we got Picture and Sound until you change the Video-Resolution in LibreElec ?
What Resolution(s) is your TV capable of ?
I think it's 1366x786 because the TV is "HD-Ready" only
Don't change it to a higher Resolution like 1920x1080 because the TV can't show it, and maybe for this reason you got an "Out of Range" message.
What Resoulution did LibreElec show to you ?
If it is 1280x720 it should be OK, and you won't get any problems, because this is a regular TV Resolution.
So if everything is alright I wouldn't change the Resolution.
But for example if some Parts of the Picture are missing or the Picture is stretched we must search for another solution.
If you want you can try this
In Settings -> System -> Display
1. Set "Resolution" to ether "Automatic" your native TV Resolution or 1280x720
This will only affect resolution of the User Interface and has no effect on playback.
If it is available I would set it to "Automatic"
2. Set "Use Fullscreen Window" to "ON" (I think this is done already)
3. Make a "Whitelist" and set it to 1280x720 60.00Hz and 1280x720 50.00Hz only
The LiberElec (Kodi) Videosettings are described here:
Settings/System/Display - Official Kodi Wiki
At last simple a Explanation (I hope I did get it right):
If some AV-Components are connect with a HDMI-Cable and are switch on, at first they "communicate" with each other "telling" the other side what they are capable.
Among other things the TV tells the other Side (for example a Blu-ray Player or even an PC) which Resolution he can show.
It's called "HDMI-Handshake", if you want more Information about it, you can Google it.
So if you turn your TV on first and then the PC, the TV will "tell" the PC of which Resolution he is capable and wich sort of Sound he can handle.