fwiw, I did a '
hard factory reset' (
method 2) on my RT-AC86U a few days ago, with Asus stock firmware (version 384.81369) which, to my delight, seems to have resolved the hanging UI issue (httpd crashing). i keep the web interface open with an idle timeout of zero (disabled). httpd crashed after few hours and always overnight: UI became unresponsive, requiring a service restart_httpd in the morning. but httpd hasn't crashed since doing the hard factory reset.
according to Asus, just pressing & holding the reset button on the back of the router may not properly restore factory defaults. a 'hard factory reset' is to: power OFF the router; press & hold the WPS button; power ON router while continuing to hold down WPS. the power LED will light up briefly then go out again, then release the WPS button. finally, power OFF the router, wait a few seconds... then power ON again. all LEDs will illuminate as the router does a 'hard reset'. a few minutes later, your 86U should be reverted to factory defaults.
this hard reset method is also covered elsewhere in the forum, but i'm posting it here as one possible solution to the hanging UI issue. seems to have worked with my RT-AC86U, so far.
oh and memory usage has also stabilized at around 300MB since the hard factory reset. before doing the reset, memory usage would increase over the hours, as high as 475MB. that was with factory defaults, wifi turned off and only 1 wired client connected for testing & monitoring.
final update: i'm sorry to report that httpd has just crashed again (web UI unresponsive).
service restart_httpd brings the UI back to life as usual. same Asus firmware .81369. after the initial hard reset, the UI did stop hanging for a few days. i rebooted the 86U last night, and now httpd is back to hanging after some random hours of uptime.
maybe i'll just keep downgrading until i find a firmware version that is reasonably stable? i mean, this is ridiculous.