2 connections (LAN via wi-fi and 3g) - why is there no Internet access when connected at the same time?

Question

Windows

There is a laptop (Samsung R525, Windows 7 x64).
2 connections are established:
1. 3g modem (defined as a connection through a mobile broadband adapter) - there is Internet access.
2. Connecting via the built-in wi-fi card to the router (Zyxel Keenetic) to the local network - the router itself has access to the Internet, but specifically for this computer in the firewall on the router there are restrictions on full Internet access.
The problem is that when connected to both networks at the same time, the computer cannot access the Internet. Not even a ping. The priority is set to the fact that the 3g modem is the first in the list.
If you turn off the wi-fi connection, then Internet access immediately appears.
Please help with advice.
PS if you connect to the router via a cable, then while connecting the connection through this cable and a 3g modem, there is also no Internet access.

Answer 1

Type route print at the command line.
When
1) the routes do not intersect
or
2) the metric of the 3G modem connection route is less (higher priority) than the LAN route, then the Internet will work.
Watch them with different combinations of connections, you will understand.
I did not find how to configure it through the Windows graphical interface so that everything works at the same time. Even if I set everything up, after a reboot or a modem departure, everything goes astray. Prescribed manually the routes necessary for work through route add after all connections. You can make the script in the form of a bat file so that you do not manually enter it each time.

Answer 2

When connecting 2 networks at the same time, you are given default routes from both gateways. Therefore, you manually need to configure routing between networks.
In this case, you need to remove the default route leading through the wi-fi router and register the route through it only to the local network. The Internet will work through 3g, LAN through wifi.