Ok I manage it and flash it via serial connection. For who is interested these are the steps.
1-Get a usb - serial adaptor (I've got this one https://tienda.siliceo.es/en/usb-serie-ttl-serial-rs232/27-usb-serial-module-rs232-uart-ttl-cable-com-port-chip-of-cp2102.html?search_query=ttl&results=15 "
2-Install the correct driver for the usb-serial adaptor and connect it to one usb port. Check the COM number via hardware properties (I think). You will need it to configure Putty.
3-Download and install Putty, download and install TFTP server;
4-To configure Putty you can use as reference this 2 youtube videos:
NOTE: just use as reference for Putty configuration, no for serial connection- please check my pictures for the connections.
Set the terminal emulator to 115200 baud, 8 bit, no parity and 1 stop bit. Do not forget to save log/sessions otherwise you will not be able to write commands
5-Open Archer C7 (remove the four screws and the upper cover (be patient...not to broke anything);
6-Now cut 3 paper clips with the same size and connect them to the router (check picture 1 and picture 2). This is a little bit tricky because I do no use soldering. You insert the clips in the holes and manage them to contact with the surface and in a way that they do not move. Server is disconnected and wireless disable.
7-I download the stripped image from http://www.friedzombie.com/tplink-stripped-firmware/ and rename to something simple like stripped.bin
8-Now I use as reference this post https://forum.openwrt.org/viewtopic.php?id=57190
In particular starting from here:
-“Run a TFTP server such as TFTPD32 on 192.168.1.100, and make sure the firmware image is in the TFTP folder.
Now, you should already be connected via serial. Use a terminal such as PuTTY and connect it to your router, using the settings from the guide I linked.
Press the power button on your router, and repeatedly press "tpl" to halt the boot.
You should now be at the console. If you can read but can't write any commands, make sure flow control is off.
Finally, run these commands.
tftp 0x81000000 [name of your firmware file].bin
erase 0x9f020000 +f80000
cp.b 0x81000000 0x9f020000 0xf80000
The moment of truth:
Your router should reboot with the new firmware. Now you can flash LEDE via web page.
I’ve tried normal LEDE and optimized version for C7 v2 and revert to stripped firmware. Everything went well.
Regarding LEDE I have an issue that is the blinking leds of the router. In router web page there is the configuration option for Leds lights but I cannot manage how to configure it to blink property. For example system light is always on, if I tried a different configuration sometimes all light are on! The router is working great but I cannot figure out he led configuration. My router lights are green but in configuration is mentioned blue!