Question about TP-Link Deco X50 (3-pack)

Yes, you can use a Deco X50 as a WiFi repeater without an Ethernet cable to the main router. The Deco units connect to each other wirelessly using mesh technology to extend the network, as indicated in the official product description, which mentions "wireless connections and optional Ethernet backhaul". This forms a single network with no signal drop-out on the move.