Also, in addition to what @Honey_Patouceul is saying, you can set up any Linux system to assign an IP address as if it is a router…without the internet. You still have the RPi and TX2 sitting near each other with a cable between them. Ethernet does not require internet.
If you are using the TX2 development kit, and using a fairly recent release, then by default there is an example virtual ethernet device over the micro-B USB cable. Keep in mind that gigabit is far faster, and also keep in mind that the dev kit ships with a micro-B cable (versus micro-A…the On the Go/OTG port accepts either), and on these more recent releases the Jetson has address 192.168.55.1, and any host having the full-sized type-A end of the micro-B cable connected will see the Jetson as a virtual ethernet device and will assign to the host end address 192.168.55.100. You’d simply tell your host it’s ok to use the 192.168.55.100 network device, and you have ethernet (it may be over USB, but the host treats it as ethernet). Ethernet is not equal to Internet. Ethernet is a protocol/standard, and will work with or without Internet (you just need both ends to have a compatible address).
Try using the micro-B USB at the Jetson end, and then the full-sized type-A USB at the RPi. On each end run “ifconfig” (or as a filter for less output:
ifconfig | grep '192.168.55'