I'm proud to celebrate my first day with OpenWRT working
I soldered some pins on the rx/tx/gnd pads, plugged the pins in that UART/USB adapter thing, launched that "tera term" other thing, preyed the gods (all of them), and hey ! I got a terminal working !!!
After beeing able to talk with that Kimax box, I could not manage to change the password of the Kimax system, only the password from a "recovery system" ? (I got a /etc/shadow and a /rom/etc/shadow in read-only, and they was different). My account also was "admin" instead of "root". Though the passwd command worked, I still was not able to log with LuCi or SSH (even with mount_root command)...
After doing a "umount /overlay && firstboot && reboot" a couple of times, trying to hit "f" then "enter" when the box was telling me to do so, and nearly thought the box was dead cause it was booting in loops, I finally managed to land in a shell with a "root" account. I changed the password, and allelujah, I could log into LuCi !!!
So then, I decided to have a meal. LoL
Right now I'm playing with openWRT and trying to make everything work (I am trying to make exfat work ^^, but that's a different story... from what I seen, I must compile it, but I'm still searching what package to compile. Must it be a kmod ? whatever, I will potentially ask in an other thread if I can't find by myself)
Thank you all for helping me, so now I can play with OpenWRT, and even talk to some electronic devices in serial mode with an UART cable !!! I had a lot of fun
And thanks to @anon69880279 for having shared his firmware. Even if it may have a login issue... but maybe I am the issue
||OpenWrt SNAPSHOT r6567-db893ec7f0 / LuCI Master (git-18.088.49307-90ed423)
||Thu Jan 17 00:15:19 2019
||0h 34m 18s
||0.54, 0.15, 0.05
||93748 kB / 125588 kB (74%)
||89104 kB / 125588 kB (70%)
||4644 kB / 125588 kB (3%)
PS: for those having the same problem, I did not found the cause, but a solution: get an UART/USB adapter connected to the board, and change the password with passwd (I had to try different booting options to get a terminal working for my actual openWRT and not a "recovery mode environment").