Change MTU on RT-N66U

[thedeadzone]

So I am trying to change the MTU for the Switch on the RT-N66U in AP mode.
But I can't find the setting anywhere in the setting pages, in router mode its located under lan but in AP mode nothing is present there.

I tried changing it with run cmd and ifconfig and changed the mtu of et0-4 and vlan1 to 9000 but when i try to ping from my windows pc with enabled 9k frames I still get the error that the connection doesnt support 9k frames.
I also tried to change the MTU of the br0 interface but is seems like its not possible.

 

[r00t4rd3d]

What happens if you change the MTU in router mode, while its visible, then switch to ap mode? I am going to guess that the router in ap mode lets the main router decide the MTU thats why its not visible.

 

[thedeadzone]

So I switched to router mode, disabled jumbo frames which apprently was already activated and switched it on again.
Running ifconfig via runcmd still shows all interfaces run with 1500mtu

test with ping -f -l 9000 routerip still shows :
Packet needs to be fragmented but DF set.

chaning the wan mtu does change the mtu of eth0 to 9000

changing the mtu on my isp router is not possible because it doesnt support jumbo frames.
***EDIT**
changed the mtu via ifconfig and found out windows is very strange about setting mtu. With right settings in windows everything works.