RJ45 connector on breadboard
I have seen ready-made "breakout boards" being sold for RJ45 connectors (RJ45 connector + header pins on 0.1" pitch already fitted to PCB). Search the usual places.

Alternatively, use hot glue to mount the RJ45 socket on a small piece of normal 0.1" "perf board" prototype PCB yourself (for mechanical stability). Then solder wires between the RJ45 pins and header pins which you have mounted in the "perf board".