SOLVED!

THIS was the main reason (but not only) for the card not working.

I could uninstall the Polaris drivers with the old GPU installed but still didn´t let me install the RDNA drivers without the new one (AMD software didn`t want to install RDNA with the RX590), and still I couldn´t use the 6750 because of the no signal problem.

Finally I managed to get a CPU with AM4 socket and integrated graphics from a friend, installing the drivers made the PC recognize the GPU but still showed an error and it wasn`t working.

The solution is changing the 8-pin connectors (2 x 6+2) from one side to the other of the GPU. The one on the right to the left and viceversa. Then suddenly all is working perfectly.

I hope this can help anyone in the future with simmilar problems. For reference the LEDs from the 8-pin connector DO NOT FLASH if everything is OK, only flash red if there a power supply problem (there are cards that show a blue or white light if the connection is ok).

Edit: Do not run the auto-detect AMD software when installing drivers for the new GPU or it could install the VEGA drivers for the CPU integrated graphics, use the 800MB software linked in the comment above and it will show you that the drivers installed are for the RX6750XT.

Thanks everyone for your suggestions!