Hi everybody!
I'm a italian student of computer science and I would try OpenWrt on my old modem/router.
My old modem/router is "D-Link 2640B B2" and i have seen that it is in the hardware list!
I have downloaded "openwrt-DSL2640B-squashfs-cfe.bin".
Then I have downloaded  and installed "Tftpd64" on my Windows 10.
I have connected my D-Link router to my PC with serial connection:
I use usb ttl converter  to use the "Serial port pinout" and I think that it works becouse I can type commands in D-Link 's CLI (I use Putty to work with CLI).

So, I tried to download OpenWrt bin file to the router with the command

cfe> flashimage ip_of_tftpd:name_of_file.bin

but it did not work!

I have tried to upload this file:
openwrt-15.05.1-brcm63xx-generic-DSL2640B_B-squashfs-cfe.bin
openwrt-15.05.1-brcm63xx-generic-DSL2640U-squashfs-cfe.bin
openwrt-DSL2640B-squashfs-cfe.bin

ip_of_tftpd what is it? I can put the ip of PC's wireless NIC or Ehternet NIC?
It have tried different IP but it didn't work...
Is the problem the file that I could try to upload?

The error message is:

Loading failed.: CFE error -21
*** command status = -21

Sorry for my bad english!
Thanks!