I didn’t change the ino file and mines been running happily for weeks!
I’ve used the Adafruit Feather HUZZAH ESP8266 on a number of projects and they’ve been trouble free (Adafruit makes good stuff), but that doesn’t mean that you couldn’t have received a bad one.
Sounds like you’re getting it programmed ok and SmartThings can find it, but looses it every 48 hours or so, right?
Once the SmartThings hub finds the ESP8266, via it’s IP address, the IP address of the ESP8266 can’t change or the hub will loose track of it.
My first guess is that the IP address for your ESP8266 is changing because it’s “DHCP reservation” is expiring.
Have you programmed the ESP8266 with a static IP (have to change the ino), or better yet, reserved it’s IP address on your router (which most likely acts as the DHCP server that assigns dynamic IP addresses on your local network)?