Been awhile since i tried asus fw , just flashed RT-AC68P_3.0.0.4_378_4585 with a hard reset (WPS) and found out MTU settings disappeared from that page WAN page!!! Is it just me or been like this since the beginning ?
i always use Rmerlin's firmware so i cant tell. I think when it comes to consumer the idea is that the less options you give the less likely they will screw up. It could also be that different MTUs may be buggy with the router/firmware.