@mboz 

Hello,

I am assuming that you are looking to use your 1 Deco X-50-PoE as a router and want to power it with an ethernet cable instead of the power supply? 

Theoretically, the following setup should work:
 

The device you need is a PoE Injector such as:

https://www.tp-link.com/us/business-networking/accessory/tl-poe4818g/
 

The PoE Injector will need to get plugged into a power outlet next to the modem. It has 2 Ethernet ports, one labeled Data and the other is labeled Data+Power.

Plug the Ethernet cable that comes from the modem into the PoE Injector port labeled Data.

Plug the Ethernet cable going to the Deco X-50-PoE to the PoE Injector port labeled Data+Power.

Hope this helps!