Trust me when I tell you that this is a flawed secondary plan. Above 2-3,000 ft AGL and at 100+ knots it's likely to just not work - and you will be distracted trying to figure out why, instead of flying the plane. Get a backup transceiver and a way to connect it into your plane's VHF antenna(s).
Aviate. Navigate. Communicate.
And review those lost comm procedures periodically.
What Dave said.
Anyway, part of the technical reason why this wouldn't work is inherently how cell phone antennas work (this could also apply to WiFi routers, and TV station transmitters).
To put it in simple terms, think of a tent, or better yet, a teepee. From the highest point of the teepee, the further down you go, the further out it goes. That applies to the range of a cell phone tower; from its highest point, the further down in altitude you go, the further out the range of the signal expands.. That's why you'll normally find those type of towers (especially TV transmitters) in the highest point of the area you're in, but they are always posted pretty high.
Since you're flying, you'd be above the antenna and its range, so you'd lose signal very fast, if you get one at all.
Coincidentally, VORs work the exact opposite. They are closer to the ground, but they are set up more like an upside down teepee: its range goes up and out, which is why you can pick up a VOR's signal from a good 100nm distance, if not longer.TL;DR:
you won't carry that long of a signal to even use a cell phone for lost comms. Better to have a backup radio or handheld, and concentrate on flying the plane, and lost comms procedures.