Due to the way the antenna works, the correct angle must be ensured when programming the DT400 (see video). The antenna is only ready to receive data transmission from the MDU in the lower left quarter of the round reader.
If the MDU is not held at the correct angle towards the reader, the MDU will return the error “Terminal offline” because it was unable to communicate with the antenna. In this case, it may help to position the MDU at a different angle in front of the reader and retry the programming.