How to reset to default firmware on AR750


I just bought an AR 750 to start playing around and I have updated the firmware, and now I am not able to go back to default firmware which I like much more. I have downloaded all firmware version on the web and updated with no success. The one running now looks like this:

and if I try to go to the address I get a 503 error, whereas now I have to go to to acces the router with this firmware. Probably it is something stupid but I am a noob on this field.

Thanks in advance

Follow the guide here:

Using the latest firmware from here:

I have done that already with no success, it does not let me access to the firmware update page ( even changing my ip to . My question is, what happend is that there was a problem during the update of the firmware, or is the firmware installed that look like that?

I think you want this: