I've changed ISP and am using their router. I wish to donate my old NETGEAR router still with its original Network Key (password). Would someone obtaining this router somehow get access to my computer files and data even though I am no longer using that network or ISP? Thanks.
If your computer is not connected to the router, other people having access to it won't be able to access your computer through it.
You could pose a convoluted (and highly unlikely!) scenario where a three-letter-agency recorded all your wireless traffic, then buy your router to attempt decrypt the messages that were sent in plaintext (ie. using only the Wi-Fi encryption) through that router in the past once they learnt the password you were using. This would require that they had recorded the initial 4-way handshake for that session, which requires being highly targeted. This would not allow decryption of anything you hadn't sent that session, or that were separatedly protected (eg. inside a HTTPS connection).
No, the only thing that they might get access to would be any saved configurations on the devices,
But aside from that your new router and all devices behind it are not connected in any way.