The RT-AC86U is a different router than the RT-AC68U. There could very likely be different reasons why its not working for you versus someone with a different router having trouble trying to update their firmware. The RT-AC68U has it's own set of issues related to limited NVRAM that can cause problems with later Asus-Merlin firmware. I had no problems installing and running 386.11 on an RT-AC68U. As always YMMV.Stick to 10, 11 didn’t prove itself on my 86U.
A suggestion. If the NVRAM is low, that may cause weird issues. Check the NVRAM value and if low try doing a hard factory reset to see if that frees up some NVRAM. If it does one can then try to update to 386.11 (follow the first post update directions for the AT-AC68U).I just tried to update my RT-AC68U to the most recent version RT-AC68U_386.11_0. It tells me to please wait until it restarts and is still Current Version : 386.10. I have tried from a couple different browsers without luck. Any thoughts?
Other troubleshooting steps. Disconnect the USB drive, if one is attached to the router, before attempting to update the firmware. Make sure to use a computer connected directly to the router using Ethernet when updating. Temporarily disable any browser extensions and security/antivirus software. Double check you have downloaded (and extracted) the correct firmware for the router.