I'm not too well-versed in hardware - is it really that easy to swap out a 2G modem with a 4G modem and have it "just work" without touching the drivers? Even if the baseband/modem chips miraculously do conform perfectly to some I/O protocol at the hardware level despite being multiple generations apart, wouldn't the difference in timings break whatever firmware the Nokia 5110 has, which was expecting only a single v…
This would be like plugging your monitor and keyboard into a new computer.
There are some lorawan handheld communicatiors using surplus blackberry cases and keyboards from blackberries that never were, that they got for pennies.
Except this dude wants to drop in a roll-your-own cellphone board with a basic os, instead of a lorawan radio and basic os. Same idea. But he will have to design this.
Whereas You can get schematics to build a lorawan communicator with blackberry parts, and there are community supported roms for that