Try it out!
I’d suggest just twisting the cable wires together instead of soldering while you run your tests. You can do a bandwidth test of single pair upload and download from the companion web interface, and see how that compares to when the wires are twisted together. It should be net less resistance, but may end up with slightly more noise on the signal, so not certain how it’ll go.
The fathom boards are signal boosters that increase the voltage so less current travels through the wires, which reduces the signal loss. Most likely a normal Ethernet signal won’t have fun going 100+ metres over wire, but if it’s not too much of a pain to connect up appropriately then you can try it - just don’t expect that one to work well, if at all.