Joe, your phone can act as a mobile hotspot, and it sounds like that's what your computer's tethered to. You can arrange for your printer and other devices to use it. It uses a load of battery power, though. It may be better to get a mobile hotspot from AT&T. That's what I have. (My printer uses it, too) Your rate shouldn't change unless you increased your data allowance, exceeded it, or received phone calls for which there's a charge.
I have a cell phone, mobile hotspot, with an unlimited plan, for under $200. I don't use my land line, and need to ditch it.