Can’t set up my extender netgear AC1750 Ex6400v3

Manuals are always a good place to start.

Visit the support pages:

Support | NETGEAR

Feed in your model number and check the documentation for your hardware.

Look for the model number on the label on the device.

That page will also have any software, firmware and drivers for your device, if they exist.

Begin with the Quick Start/Installation Guide.

There may also be some helpful videos on what to do.

You may have done that already. I can't tell from your message.

I mention it because Netgear gave up on supplying paper manuals and CD versions some years ago and people sometimes miss the downloads.

Just another user with time on their hands.