you have to set network settings to manual instad of DHCP. (libreelec-settings)
had the same problem. i solved this with manual IP-Settings
It worked!
Just a note for people with the same problem, I have to connect to internet first for the connection tab to show stuff. Also, the Tvheadend seems to boot a lot slower when booted offline. It took me about a minute for the TV tab to pop up.