Well, in theory this is possible, OBS has the option "Bind to interface" in the advanced settings. So you can tell it to use either your mobile line or wired connection. It can be a bit complicated to send all other stuff over the other connection though.
Windows gives you the option to "order" or prioritize the connections in the advanced network settings, you will have to google that. Recently a user also mentioned a software that can help with this process. Maybe you can find the thread on the forums.