I found a temporary solution (passive antenna hardware under $100).
Limited to 1 very specific location (semi-moveable), but it solves the main problem: ZERO cell signal in rural basement office/workshop.
So, it might enable me to really start to use UT:
I installed a directional outdoor cellular antenna ($30 ebay) on a pole outside, aimed it generally toward a cell tower (based on map search), connected 50 ft (17meter) of cable (single piece of coax), then indoor antenna ($12).
No amplifier.
Indoor antenna is mounted horizontally on a plastic base a few inches (75mm) above office desk (formica surface, on sheet metal frame), and set cell phone (Nord N10) on top.
Phone now has enough 'bars' (3 or 4) to use voice calling and exchange SMS.
Location is open workshop / office, so I should be able to hear ringing and most notifications.
Preliminary testing, but initial results are encouraging.
Only issue is, has to stay VERY close to antenna (and orientation probably matters. Not tested yet).
I mention this solution because a no-amplifier system might work for others, and is fairly cost effective.
I might still order a used amplifier, which would enable moving phone away from the antenna (to anywhere in room).