ADB Fastboot
// Top of ur .cs file
...
using AndroidCtrl;
 
using AndroidCtrl.ADB.Binary;
//or
using AndroidCtrl.ADB.Socket;
...
 
// Reboot
ADB.Instance().Reboot(BootMode.REBOOT);
 
// Reboot-Bootloader
ADB.Instance().Reboot(BootMode.BOOTLOADER);
 
// Reboot-Recovery
ADB.Instance().Reboot(BootMode.RECOVERY);
 

NOTE
If u have more then one connected device and no device ID is selected in the class (so the base instance is selected) u'll reboot all connected devices! So it's better to call
ADB.Instance("device ID").Reboot(BootMode.REBOOT);
 
Fastboot.Instance("device ID").Reboot(BootMode.REBOOT);
if no device is selected.