SomeWhereOverTheRainBow
Part of the Furniture
I highly recommend @L&LD 's guides for a successful setup.@Kenji you may want to fully wipe the router (M&M Config and Nuclear Reset), at least the JFFS partition and your USB drive before you begin again.
The following posts may be useful to you to get a good/known install of the firmware and amtm + USB drive format + swap file + script(s).
https://www.snbforums.com/threads/ax88-packet-loss.62891/#post-563326
amtm Step-by-Step https://www.snbforums.com/threads/amtm-step-by-step-install-guide-l-ld.56237/#post-483421
M&M Config https://www.snbforums.com/threads/n...l-and-manual-configuration.27115/#post-205573
WPS NVRAM Erase https://www.snbforums.com/threads/b...eta-is-now-available.55520/page-9#post-473141
Nuclear Reset https://www.snbforums.com/threads/major-issues-w-rt-ac86u.56342/page-4#post-495710