Netgear D1500 300 cant reach 300 why ?

Hello 

please i have 2 Qs

first one i update firmware directrly from 1.0.0.15 to 1.0.023 without downloading 1.0.0.18

second Q

i cant reach the full speed 300mb

inspite of changing and trying all channels from 1 to 11

i made the transmition to highest level 

and i made the # to 300 in router setting

i dont know whats the problem