I'm seeing very similar / identical issues on Pi 4. After initial installation, could join WiFi network, and even used it to transfer quite a lot of data.
But at some point (later that same day), it becomes disconnected, and impossible to reconnect. The only solution I've found thus far is to completely re-image the SD card with a fresh install of LibreELEC!
The SSID still shows up in connections, but every time I try to connect once it's stopped working, it just gives various errors and sometimes even reboots LibreELEC!
I tried setting `options cfg80211 ieee80211_regdom=US` in /storage/.config/modprobe.d/cfg80211.conf and thought that had fixed it, but eventually the WiFi connection still went bad.
I've got it working again today, and trying using manual configuration instead of DHCP, but I doubt that's where the problem lies... We'll see how long it remains connected this time...