In Windows, there is a list that determines which network connection gets priority for network services. Perhaps you have the Modem set above the LAN connection?
LAN - access to all company docs, info, shares etc.
BB - access to restricted stuff.
you can only prioritize your networks. there is no option to have both, because it's not a feature that is needed 99.9% of the time. the system can detect when a connection is not working and use the other one, but it has no reason to use two at the same time (and I imagine it would cause a great deal of issues because how would it determine which network gets what data?)