This Arduino "sketch" is a framework for using an Arduino as a KISS-TNC. There are other TNC implementations but what makes it different is that this software is for transceivers like the rfm95w LoRa module or the nrf24l01 and many others (and not for transmitters).
You can find it on GitHub.
Make sure /etc/ax25/axports exists. In this file you need to have something like this:
ax0 FH2LOR-1 9600 254 1 AX.25 over LoRa
After that, "start the network device" with:
/usr/sbin/kissattach /dev/ttyUSB0 ax0'/dev/ttyUSB0' may be different depending on the type of Arduino and/or tty-to-usb converter you use.
I've tested it with hoperf rfm95w lora modules and was able to send and receive ax.25 beacons. I'm currently saving money for a Moteino Mega USB device. The plan is to place a device on two locations here in the city between me and a friend and see if longer range with wording is also a workable solution. Maybe play around with netrom and rose.