By default, wehn you use mobile Internet over the router, your PC connecting to a router which has the mobile modem connected to it, Windows sees that as non-metered connection. Connecting the mobile modem directly to PC is seen as metered connection.
To manually change your home network to metered give this command in an elevated command prompt:
The parameter NetworkName is the name of your network. Notice that it is case sensitive; if your network is called MyHomeNet, giving the parameter as myhomenet won't work.
netsh wlan set profileparameter name="NetworkName" cost=Fixed
You can toggle the connection type with above command:
- Metered connection: cost=Fixed
- Non-metered connection: cost=Unrestricted
See the tutorial: Wireless Network Metered Connection - Set in Windows 10 - Windows 10 Forums