I tried to return to stock firmware however none of provided solutions work.
Fristly I followed https://openwrt.org/toh/asus/rt-ac58u#returning_to_stock but after step 6 router is not in recovery/rescue mode, so I can't execute step 7 and 8, because restoration tools can't see router that is not in recovery mode. Interesting thing is that after step 6 router is rebooting, but it's booting up back immediately to normal OpenWRT system.
I tried as well https://www.asus.com/support/FAQ/1000814/ and it shows:
However router after restart boots back to OpenWRT.
Any ideas how to get original firmware?
Thanks in advance!