I have two network adapters: one for my normal LAN and my tethered phone as I don't have regular internet access here yet. When both are plugged in, KDE/Network Manager prefers to route all internet traffic via my home LAN, indicated by the blue "earth" icon and the fact it doesn't work. How can I configure this?
When you set up your phone as a network device you should get in your OS a device akin to
phone0 which may or may not have an IP address on it.
When this device come online you should make it default route for all traffic.
sudo route del default sudo route add default gw phone0
Devices on your local network will still be accessible since route to that network will still exist as far as every other destination is concerned it should to through your phone.
One thing I cannot be sure of is how this can be done through the network manager but you can figure it out.
P.S. Just make sure your phone is not connected to the net through WiFi of your LAN.