Wow, that Spark thing is... sort of creepy the way it ties you to a third-party web API and servers that could go away anytime and basically says "trust us!" when it comes to security.
In any case, it's just a small microcontroller connected to a module with a custom firmware that instead of offering "serial bitstream-centric" access to a UART port it gives you an interface to twiddle GPIO pins through. (Which is actually counterproductive in this case.) There are lots of serial-centric modules floating around that would probably do (just a sample) for setting up a PPP network connection to an appropriately configured server computer. (I've read the data sheets for a few, in general you treat them sort of like a modem. What you'd need on the Duo end is a PPP dialer that allows you to set up a very-custom dialer script.)
The "holy grail", of course, would be a small embedded computer that was powerful enough not to just tunnel serial over a TCP port, but to act as the PPP endpoint and to "proxy-nat" through a WiFi connection on the other side. (thus dispensing with the need for a "support server".) I'd guess that in fact the embedded CPUs in most of these modules could in theory handle that but I doubt any of them supply the source code necessary to hack that functionality into them. For that you'd probably be stuck looking for the smallest embedded system that runs Linux (or something) that has a serial port and a WiFi chip. I dunno how much space you have to work with, but there might be some candidates out there.
In any case, it's just a small microcontroller connected to a module with a custom firmware that instead of offering "serial bitstream-centric" access to a UART port it gives you an interface to twiddle GPIO pins through. (Which is actually counterproductive in this case.) There are lots of serial-centric modules floating around that would probably do (just a sample) for setting up a PPP network connection to an appropriately configured server computer. (I've read the data sheets for a few, in general you treat them sort of like a modem. What you'd need on the Duo end is a PPP dialer that allows you to set up a very-custom dialer script.)
The "holy grail", of course, would be a small embedded computer that was powerful enough not to just tunnel serial over a TCP port, but to act as the PPP endpoint and to "proxy-nat" through a WiFi connection on the other side. (thus dispensing with the need for a "support server".) I'd guess that in fact the embedded CPUs in most of these modules could in theory handle that but I doubt any of them supply the source code necessary to hack that functionality into them. For that you'd probably be stuck looking for the smallest embedded system that runs Linux (or something) that has a serial port and a WiFi chip. I dunno how much space you have to work with, but there might be some candidates out there.