Let’s fix one problem at a time first: connecting to APs.
The GUI isn’t always descriptive when there’s a problem connecting to a WiFi network. If you’re familiar with the command line, you could try connecting via that. See Could not connect to Wi-Fi - #16 by DBMandrake.
Any errors will be a lot clearer. OSMC won’t connect to WiFi on reboot if an Ethernet cable is attached. This is intentional.
Tethering is a bit hit and miss with ConnMan. It may be an idea for us to use hostapd or something else in the future.
Sam