Have a Mi 4A Gigabit running openwrt 21.02 in bridged mode (all interfaces are one bridge and just passes through all DHCP broadcasts to all interfaces - the Wan interface is connected to the lan switch).
Following a faulty bulk update of packages that ended up in errors about libc and other linked libraries, the router is not coming back.
Specifically the wan physical interface when connected brinks the blue light on, but the orange light flashes at about 2 flashes per second.
What does this amber flash means and what are my options for reviving the router?
You mean you just told opkg to install every updated package in the repositories? If so, you'll need to reset your OpenWrt-installation to defaults, because you're not supposed to just nilly-willy upgrade everything.
Under normal circumstances, you turn on the 4A, wait for leftside blue power LED to appear after about 1 minute.
Then press and hold the Reset button. After about 7 to 10 seconds, the power LED will change to rapidly flashing yellow light. Release the Reset button.
Eventually the 4A will reboot and you can then log into openwrt at 192.168.1.1
Yes, so you did exactly what I asked you about. Next time, don't do that; there sometimes may be an updated system-package there that when installed breaks OpenWrt. OpenWrt's rootfs is read-only, so any packages you upgrade don't actually modify rootfs, but instead end up in the overlay-partition; if the package is one of the system-packages, OpenWrt will boot with one version of the library/libraries from the rootfs, but then once the overlay has been mounted, any newly-run processes will attempt to use the new library/libraries and you end up with a mismatched system.
It's best to only upgrade specific packages you're experiencing some problem with, or just wait for the next point-release of OpenWrt and install that instead.
It was faster to buy a new Mi 4 router then revive the old one.
So openwrt was installed like a charm, started configuring the interfaces, made a mistake in the configuration and got locked out (due to ending up having no interface configured).
What is the solution?
Reset.
What is the result of resetting?
BROKEN ROUTER!
Orange light is blinking.
Blue light on when connecting a cable on the WAN interface, but no IP related traffic from the router (confirmed via whireshark).
Could there be something wrong in what happenes during updates/resets that breaks the router?
If you have mis-configured the network but then do what @bill888 said, it should reset OpenWrt cleanly to defaults. If that does not work, it's a bug.
If the left light never changes to solid blue, OpenWrt has not booted fully and an OpenWrt reset is not going to work. In that case you need to use the TFTP recovery to stock and then reinstall OpenWrt.
For the 4A, turn on the power, a solid yellow power light will appear. A few seconds later the yellow light will start to flash, press and hold the Reset button until you see the power LED to flash VERY rapidly. Then release the button.
Connect PC to LAN port closes to WAN port. SSH into 192.168.1.1.