If DYNAMIXEL servo is not detected or some of the servos connected in a chain are not detected then you may fix the issue by doing a firmware recovery using Dynamixel wizard 2.0.
IMPORTANT Make Sure you connect 1 motor only at time for firmware recovery.
Connect servo over U2D2 to Dynamixel wizard.
In wizard go to recovery option
Select your motor model
Chose your firmware
Walk through the remaining steps (it will ask you to unplug the servo power for few seconds).
- On completion, setup you ID and baudrate as per your requirement because firmware recovery set everything to default settings