Earlier quoted context omitted.
There is, it's called a USB 4G modem that plugs straight into whatever server you want.
Many carriers don't allow non-authorized devices like 4g modems to send SMS on their networks now. That's a recent change (last year).
Not a great solution, I guess, but ... zip-tie/hot-melt the hardware into a box with a fan and I guess it'd be mostly as reliable as a Raspberry Pi?
Is there a cheap Android phone with an external antenna socket still?