Low speed issue with N600 WNDR3400 router and Arris 6141 modem

i have a iphone 6s,too,i cant get high speed,I have a Comcast Xfinity connection that gives me download speeds of close to 70 Mbps.With a wired connection directly from the modem I get the high speeds .However, with the WiFi connection through the router I get the download speed (~25 Mbps).how to do it?