D6200 wan 1gbps

Hello guys. i'm upgrade my connection from 100mb\s to 1000mb\s whitout router (d6200) speed up to 900. but with router only 70-99. 

Cable from fiber modem to router 8 wires (1gbps ready) and from router to PC 8 wires to. but why router dont want take that and used only 100mb\s ? =(