I joined the forum just to be able to reply to this thread. I use Libreelec version Generic.x86_64-10.0.2. I think I'm lucky because I solved it in 2 ways:
A - I connected via USB an old mobile phone (NGM Forward-Next still in my heart, but no longer usable) with Android 4.2.2 and I activated USB tethering. In my phone you can reach the menu in this way: settings/advanced wireless settings/tethereing-hotspot/usb tethering.
Nothing else to do. Libreelec consider it as a cable connection, in fact it doesn't ask for SSID or Password, it simply connect to the LAN.
It works perfectly, but every time I turn on the mediacenter I have to reactivate the tethering of the phone (it's just a tap, but I'm too lazy 
).
B - I bought a M-TK moveteck model GT837 key which in the package says it is supported by linux. I connected it and it works, nothing else to do. Obviously from the Libreelec settings you need to indicate the SSID and password of the WiFi you are connecting to. Only one attention, If connected to USB 3 (the blue usb port) it is much slower. I don't know why, so use the USB with black color.
If some expert can explain better it would be interesting.
The speed between the 2 methods is exactly the same.
I hope this post could be useful