To update this, I've managed to get this working, but I don't quite understand what's going on yet! So, I tried running LFS as an admin. Confusingly, it seems to be using the cfg.txt in the install folder now, but what cfg.txt was it using before?!?!?!? Evidently it's a different one, somewhere else...
I've gone through and completely opened 29999 and 30000 on both inbound and outgoing connections in Windows Defender, as well as opening all ports for both the LFS and Fanaleds executables, no luck.
Raised this before on LFSForum but still having no luck. Fire up fanaleds, fire up LFS, /insim 29999, and in game it says; InSim : port 29999 InSim - TCP : Fanaleds 2.3i InSim : version 0 requested - using 6 InSim guest closed : Fanaleds 2.3i In Fanaleds we have; Status: Running Game: Live For Speed...