Thank you!
A few notes:
fw_setenv flag_last_successshould be0forfirmware=1and/dev/mtd8as shown here. With1I got bootloops.- I was able successfully install OpenWrt U-boot as described here.
No moreflag_try_sys1_failed. - It's possible to access UART pins without opening the router, though the holes in a case. I realized that after opened it =) . Hair dryer will help not to damage sticker/label.
sysupgradewill reboot the router automatically.
@zeluis9 , try fw_setenv flag_last_success 0