This post somewhat follows up on the tethering struggle.
My mother also struggles to maintain connection on her cell phone in the upstairs bedroom. The router is directly beneath her bedroom, so there's probably too much plumbing and wiring in the way that interferes with the signal.
I recently serendipitously found a steal of a Nighthawk EX7000 for cheap on Mercari. These things are MSRP $170, and I got mine for $40 including shipping. What a deal.
After setting it up (which came with its own struggles, namely that the seller didn't give the device a courtesy reset), I've positioned it right outside my room's window. It comes with LAN ports which I conveniently wired to my desktop. Now there's no more tethering, I don't have double NAT, the connection speed is amazing, I don't randomly drop connection anymore, and I am part of the rest of the home network. I can even stream and play multiplayer games without worrying about random drops.
My mother also has the added bonus of getting much improved connectivity in her bedroom now. From taking ages for one picture to send to sending them all in quick succession, the phone now keeps my mother from leaving her room in the morning.
Overall, an excellent product that does what it advertises it can do. I've already gotten return on investment.