The best solution is actually sending a low voltage AC or DC coupled with a "Bias T". A Bias T is an inductor with a capacitor that allows for mixing AC+DC signal in a wire pair. Bias T are simple, and very cheap to make or buy. Low frequency Bias T are the solution to add power through the tether cable and being able to transmit the Ethernet commands to the unit.
Gaston T., PhD Physic (with patents applications in Multimode Comunication and plenty of articles of fiber optics communications). Will send by request.