Problem is, not exist official *.trx firmware for web browser client.
I do not understand with methot with bootloader
" Method 2: via Bootloader and an Ethernet port "
I don't know from get IP address of the bootloader if exist.
I tried something like this
$ /usr/bin/tftp (to) (here is router IP) tftp> put openwrt-18.06.2-ipq40xx-asus_rt-ac58u-initramfs-fit-uImage.itb tftp: sendto: Operation not permitted tftp>
Maybe exist way rebuild *.itb to *.trx? (on own PC)