You can directly plug your other router into one of the Starlink Gen 3 router's Ethernet ports. The Gen 3 router has two Ethernet ports, which allow you to connect other devices or routers without needing an adapter. However, if you want to bypass the Starlink router entirely, you might need to use an Ethernet adapter depending on your setup, but this is not necessary for simply connecting another router to the Starlink Gen 3 router.

To the router. It is not very long

Preferably a normal Lan cable with up to 1Gbit/s performance. I personally use a Cat 6 but Cat5e should be sufficient for Starlink. I can recommend the Digitec Ethernet patch cable which works perfectly for me.