Your computer route network traffic based on a routing table. Usually, Ethernet is expected to be the shortest path, WiFi is the next shortest path, and then mobile broadband devices. It's so that you get the fastest connection when plugged in with Ethernet cable, and you don't get charged for mobile broadband when connected to WiFi.
In your case, the interface that best serves the "Internet" (i.e. any address that isn't local) is WiFi. To change this, you can manually set the interface metric for your adapters so that mobile connection has shorter interface metric than WiFi. Step by step:
- Right click the wireless icon in your notification area and click Open Network and Sharing Center.
- Click Change adapter settings
- Right click your WiFi adapter, click Properties
- Scroll down the list and uncheck Internet Protocol Version 6 (TCP/IPv6)
- Scroll down the list and open the Properties window for Internet Protocol Version 4 (TCP/IPv4)
- Click Advanced button
- Uncheck Automatic Metric, and enter 50 in the Interface metric
- Click OK enough times to save your changes to the WiFi adapter
- Do the same steps 3-8 for your Mobile Broadband adapter, but this time, set Interface metric to 40