Arduino nano not working properly on external source

I have been using an Arduino nano (not original) to make some prototypes tests of a project with a few buttons and 4 7-segment led display.
When i plug the Arduino to te usb cable everything works fine, however when I try to connect it to an external power supply it goes crazy. The L led as well as the Tx and Rx led blinks, the displays keeps resetting itself (one of the buttons was supposed to do it) and sometimes the numbers go crazy. It looks like that the power is going randomly to every pin or something.
Any advices on what to do?

Sounds like the power supply is undersized for the load and/or generally garbage, and is dropping out and/or cycling on and off when you have your project connected to it - but since you haven't posted a wiring diagram, we can't help you further, since we have no idea what power supply you're using, whatall devices you have connected, and so on.

If you're expecting to use the 5v regulator on the nano to take a 7~12v power supply and generate 5v to supply your project with, that is probably the issue.

You can also get "project goes crazy" reactions if you don't have the grounds connected (and it's possible that there could be a sneak path for the ground that you may not be aware of when it is powered from the computer).

But who knows, without a wiring diagram, it's mighty hard to give advise!