A data flow relay device includes a mobile communications device, a proprietary grand master and a transmitting and receiving device. The mobile communications device generates multiple wireless signals for communicating with a peer communications device and generates a clock signal and obtains information regarding a signal of a start phase of a frame and information regarding a total propagation delay according to the wireless signals. The proprietary grand master adjusts a reference clock according to the clock signal, the information regarding the signal and the information regarding the total propagation delay, such that a phase of the reference clock is synchronized with a phase utilized by the peer communications device. The proprietary grand master further generates a packet including information regarding the reference clock. The transmitting and receiving device receives the packet and transmits the packet to a back-end communications device.