Ethernet metric

You want the Ethernet adapter available only for configuring the long-haul router while WWAN remains the Internet path, even when the cable is connected. Two Windows behaviors matter here: route metrics choose between matching routes, while Windows Connection Manager can prefer Ethernet over cellular independently.

First, configure the Ethernet adapter with only the router-subnet IP address and mask; leave its default gateway and DNS fields empty. That prevents DHCP from adding an Ethernet default route. Then disable Automatic metric on both adapters and assign the lower metric to WWAN. Check the result with Get-NetRoute -AddressFamily IPv4 or route print; the only 0.0.0.0/0 route should use WWAN, while the router subnet uses Ethernet.

If WWAN disconnects merely when Ethernet appears, check the “Minimize the number of simultaneous connections” Windows Connection Manager policy and set it to allow simultaneous connections. On a managed device, your administrator must change that policy.