It’s a 4 year old story from nearer 5 year implementation using old firmware that is no longer maintained. The dev tools have evolved, the underlying TTN infrastructure has evolved and improved and TL:DR it’s likely not a great place to start such a development unless very familiar with paring and optimising LMIC & application code. 1st step would likely be move on to a current and supported version of LMIC… atleast 4.0 pref 4.1…MCCI lib. Or look at LMIC-node implementation as alternate start (haven’t checked if your board supported/viable) but some of the experts will know (calling @bluejedi@descartes)…
Unfortunately LMIC-node when using MCCI LMIC suffers from the ATmega328’s limited memory and won’t fit on it. Therefore for Pro Mini LMIC-node uses ‘Classic LMIC’ the now deprecated predecessor of MCCI LMIC instead.
So while LMIC-node works with and supports the Pro Mini, it will use the older deprecated Classic LMIC library to make it fit on the Pro Mini.
For new developments better use some of the more recent 32-bit microcontrollers.
Which is great, as long as the actual application you are writing is small or amazingly efficient and a GPS tracker is unlikely to be small and making it amazingly efficient is going to require you to forego the standard GPS libraries so you can parse the data stream as it comes in as holding a whole line in memory is going to hurt.
If you ONLY wanted to read a temp/humidity sensor, that would be feasible but less ideal as the classic LMIC that does fit is only just TTS v3 friendly.
So all in all, best get yourself another board - if you need it small, I hear the Teensy’s are good.