Reboot is only available when the robot IP is set to 192.168.186.2. If I set the USB subnet to something else, such as 192.168.181.2, I get the following message event though I call the API over the USB connection
ubuntu@turtlebot4-1-rpi:~$ curl -X POST http://192.168.181.2/api/reboot
{"error":"Reboot failed: must be connected via USB or SOFT AP"}