Hidden WiFi

WiFi networks with a hidden SSID are not visible to the connection manager (ConnMan) so cannot be selected in the LibreELEC settings add-on, and even if you manually configure a new ConnMan network profile via SSH, ConnMan cannot see the network to use the stored profile to connect.
LE10 and older use ConnMan to manage connections and wpa_supplicant to create the actual connection. LE11 also use ConnMan to manage connections, but switched from wpa_supplicant to iwd allowing a simple systemd service to be used for connecting to hidden networks.
Create /storage/.config/system.d/hidden-ssid.service with the following content:
Description=Hidden SSID Connect Service connman.service iwd.service connman.service iwd.service
ExecStart=/usr/bin/iwctl --passphrase <PWD> station wlan0 connect-hidden <SSID>
ExecStop=/usr/bin/iwctl station wlan0 disconnect
Replace <PWD> with the WiFi passphrase and <SSID> with the name of the network. On most LE devices there will only be a single wlan0 wireless network interface, but if you have more than one card and wlan1 or wlan2 etc. are visible, set the correct NIC/card to use.
Next enable and start the systemd service:
systemctl enable /storage/.config/system.d/hidden-ssid.service
systemctl start /storage/.config/system.d/hidden-ssid.service
Starting the service will initiate the connection and ifconfig should show an active NIC with an IP address. Ensure testing is done with Ethernet cables unplugged else the default traffic route will be on the eth0 Ethernet connection.
NOTE: Users commonly hide the network SSID to improve privacy and network security, but it is disclosed in client connection requests making a hidden network easily discoverable to anyone sniffing WiFi packets and looking for it. Hiding the SSID provides zero protection.