How do I make my computer connect to a wireless network while also receiving internet service from my phone?

[Adam]

I live outside of a big city and cannot get any sort of wired high speed internet (Comcast, AT&T, etc.) so I have to use my phone as my internet source. I could use my phones hotspot but unfortunately I am limited to 2.5 GBs of data through the hotspot. However, I can tether my phone to my PC and take full advantage of my phones unlimited data plan.

What I am trying to accomplish is, I purchased a Chromecast which is a wireless receiver that allows you to stream services such as Netflix to a TV. I have set up my old wireless router to create a "wireless network" even though I have no internet run to it and my phone with not tether with my router. So my question is, how do I connect my PC to the internet with my phone and also connect it to the wireless network simultaneously so that it can communicate with the Chromecast?
I would like to add that when I try to connect my PC to the wireless network it disconnects from my phone. That is the issue I am dealing with. I understand the order in which the internet should "Flow", I just don't know how to make my computer stay connected to my mobile when I connect to the network.