I have a rb2011 uais-2hnd-in router that has not done any update from routeros 6.30.2 - after downloads of the file and reboots it was starting with the same version. I tried with all the version, tried to downgrade and I only succeeded with v5.26. After this downgrade, I tried to upgrade to latest version (6.40) but after reboot, the router restarts continuously. I tried with netinstall in win 10, win 8 and win XP but no success, does not see it on devices.
I made some gif animation with the state of router and netinstall software.
What can I do to save this router??
Sorry but GIFs doesn’t provide enough detail to troubleshoot.
In fact the first one leads to believe a successfull netinstall was done.
Good idea anyway 
Try powering off the rb2011, press reset, and while keeping reset pressed, power it on until without releasing reset until you see it on the netinstall list of devices.
Tip: connect it directly to the netinstall computer, and disable any other network interfaces but ethernet (wireless, etc) on that computer.
It could be that the reset button is damaged, in this case, you could connect via serial to the RB2011 in order to change bootloader parameters directly from there, setting boot from ethernet (type o) right there after power on.
Typical Cisco (light blue) serial cables are compatible, so are other networking vendors serial cables.
Pinout:
Did you follow the netinstall instructions mentioned here? https://wiki.mikrotik.com/wiki/Manual:Netinstall
Downgrading back and forth to such old version is not very wise IMHO.
v5.26 is 4+ years old release! There have been tons of changes between then and now. I am not surprised that it got stuck.
Make sure, if you are able to netinstall/boot it, that Current Firmware matches Current Firmware on System > Routerboard.
In fact problem could be bootloader related, as Cha0s said, going back and forth between such major ROS versions is a recipe for disaster…
I followed the instructions from netinstall Cha0s, but my router was rebooting continuously and the router was not seen by netinstall - probably bootloader problem. Maybe it was not very wise to downgrade to v5.26 but it was to only upgrade/downgrade possible.
I managed with the console exactly as Pukkita said, so I owe him a beer.
glad it helped, thanks for reporting back success!