USB C to Ethernet on iPad Air 4 in hotel

You are getting good advice and the hotel probably needs the hardware media access control address which is like a serial number on the adapter. If you can’t find that out from a computer, manufacturer support, the labeling on the part or the box, ask if you can use a web browser to navigate to http://captive.apple.com to register.

You can also ask them if setting the DHCP client ID or address manually works. You can’t discover the ethernet MAC address from iOS 14 (afaik) - here is what you can see and control in the settings app if you tap Ethernet.

For WiFi and Bluetooth you can see these addresses on iPhone and iPad (like 16:00:DD:EE:FF:78 ) and even specify a private one different than the factory address, but this doesn’t apply to ethernet adapters currently.