If you are truly doing "device to device streaming" that should not count against your cellular data plan; and if it somehow does using the US Cellular 4G/LTE modem/router then adding another router, as you suggest, should rectify that.
However, your example--Apple TV--actually streams from the internet, right? That, as Wayne said, does count against your data plan.
However, your example--Apple TV--actually streams from the internet, right? That, as Wayne said, does count against your data plan.