OzScopes seem to have what you want, bit expensive though:
https://www.ozscopes.com.au/skywatch...rj45-plug.html
This should be easy to make up, if you have a crimp tool. This is an annotated photo of an actual SynScan RJ12-to-RJ45 cable:
https://stargazerslounge.com/uploads...bf36cd585f.jpg
This photo aligns with the connector pinout diagrams found in the Skywatcher manuals. Beware that there are threads on StarGazersLounge and here that show incorrect wiring (failing to cross the RX and TX lines). Those may or may not work, depending on the electronics on either end.