Direction on pulling data from pellet stove

Howdy, long time lurker!

I was wondering if anyone could give me a hand with trying to pull data from my Harmon Absolute43 pellet stove. Looking at the board, I see an RJ45 connector and thought "seems like a wireshark-like sniffer would do the job".

However, all searches end up looking at networking rather than simple data sniffing. Any ideas as to how to add a "bridge" that also stores the data? From there, I can do the publishing side of things, just the hardware component for me has me scratching my head.