Do you really mean "happen much"? Looks like you meant "not happen much".
Theoretically I am ready to flash, but maybe I can make a backup before. I found http://www.geektalks.org/how-to-backup-flash-from-tplink-wr703n which suggest a very simple way for a WR703N.
backup u-boot:
cat /dev/mtd0 > /tmp/backup_u-boot.binbackup art:
cat /dev/mtd4 > /tmp/backup_art.binbackup firmware with your settings:
cat /dev/mtd5 > /tmp/backup_firmware.binbackup full flash:
cat /dev/mtd0 > /tmp/backup_fullflash.bin
cat /dev/mtd5 >> /tmp/backup_fullflash.bin
cat /dev/mtd4 >> /tmp/backup_fullflash.bin
To do something like this, I have to access the C7 with the stock-fw via ssh. I am asked for a username and a pw. I tried user admin and the router PW, but I didn't get access. Maybe ssh-access has to be allowed in the stock-fw, but I couldn't find a menu whith an ssh-option. Any ideas?
https://openwrt.org/docs/guide-user/installation/generic.backup mentions a solution for the C9, but I do not have an UART console.
The method above works great, but only if you have SSH root access to you router. In some cases when you don't have SSH root access to router, but can connected from UART console. For example TP-Link Archer C9 HW ver 5.0 with original stock firmware
I don't see much of a benefit here. You can easily go back to stock via TFTP. Of course you can try to create images of your device partitions, but for me it is not worth the effort in this case.
I flashed the C7 in the meantime with success. Looks like, there is no ssh access for the user. People are reporting that ssh is reserved for the tether app.
Thank you for your help again!
Edit: I want to mention, that the flickering of my LEDs was very different than described.
- After about 7-10 seconds release the reset button. The power LED will flicker rapidly for ~3 seconds, indicating download of the firmware file.
- The router will write the firmware to flash during ~40 more seconds of occasional power LED blinks, and then will reboot by itself, ready for use.
I didn't notice the flickering for 3 seconds, I saw nothing for a "long" time, no idea, maybe 15 seconds or longer there happened nothing and then the Power LED started flickering permanently for a very long time, maybe 20 seconds or longer and then there was a constant light with the Power LED. I would say, wait for about 5 minutes before you try anything, if it worked.
this backup was very easy:
dd if=/dev/$(grep -i '"art"' /proc/mtd |cut -c 1-4) of=/tmp/art.backup
dd if=/dev/mtd0 of=/tmp/boot.backup
You are welcome and good to hear it worked out for you 
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