Serial did worked for me!
I've broken my ftdi converter in the meantime and managed to fix it as well, however one info I'd like to leave here for the others:
when you're connecting to the router using serial you also need to have lan connected (for TFTP to work).
Hi All, I've flashed my DIR-300 with the 1.5.8 and (obviously) it's not running it very well. In fact it's not stable at all. So I wanted to downgrade it to 1.4.7 (which has been quite good). However GUI upgrade doesn't work, I can't access the Bootloader, also TFTP doesn't work. I did some research...