I tried to update the firmware of TP-MR3020 router via ssh using openwrt-ar71xx-generic-tl-mr3020-v1-squashfs-factory.bin, but has not been successful. Now all leds of MR 3020 blink together. I can not access with telnet or ssh. What can I do? Thanks.

so your router is bricked now

Thank you. How can I recover the bricked router? It's TP-MR3020 v 1.9. Thank's.

Does failsafe work?

https://wiki.openwrt.org/toh/tp-link/tl … lsafe_mode

Thank's.
I tried to reset the router in failsafe mode several times, but no ping to 192.168.1.1 was successful and all leds of MR 3020 continue blink together.

You seem to have to flash a new image using the boot loader (uboot), as mentioned in the wiki. Also take the time to read through the whole page and especially the part further down regarding the serial port.
IMHO, it is less troublesome to avoid all the hassle of setting up a TFTP server and transfer the firmware image via the serial line using Kermit protocol (transfer time is about 8-10 minutes). To do so, instead of issuing "tftpboot ..." you use "loadb 0x80000000" and tell your terminal program to send the firmware file.

I own a Raspberry pi 3. It has, as you know, pins 8 and 10 at 3.3V able to transmit and receive serial data, similarly to TP-MR3020. I could attempt to connect Raspberry pi to MR3020 and run Kermit directly on Raspberry. It's possible for you?

I don't know the details of the Raspi, but would expect that its serial port works at 3.3V. So, if you can confirm that, you can of course use that, for example with minicom.