Struggling with NAS and internet connections

I have just moved and don't have a home internet connection (mostly USB tethering) and won't for about a month. 
I am tethering using my phone and unlimited data tariff, which works fine for now. I have also purchased a cheap router so I can continue using my NAS drive. I would like to be able to use both of these connections at the same time. When I bridge the networks, it occasionally works but the internet is incredibly slow. 

I have very limited knowledge in this area, but what I guess is happening is my PC thinks the cheap router is providing an internet connection, whilst it isn't, and it needs to use my phone for all of that. That could all be wrong though.

I hope that this is the right place for this, and if you need any more information or screenshots let me know.

If you could look into OpenWRT you could get a custom liunx firmware to mount your phone as a network interface.