I have the same problem I bought 2k of HAP lite move from TP-LINK just for disable the reset button and know I can't....
Can you also post what vlan is in what bridge ?
and why are you modifying the l2mtu on the bridge ?
Many thanksin v5,18 and older there is a software bug that hides the fan setting from RouterBOARD menu. try to upgrade to v5.19 and check routerboard settings, you should be able to enable the fan there.