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).
The user won't get access to anything on your computer.
However, if an attacker had recorded WiFi traffic from that wireless access point, and then later acquired that password, they could decrypt and read all the past WiFi traffic as if it had no password, so they could see what sites you visited and any unencrypted data that was sent over the network. Most sensitive websites should use encryption via HTTPS, so the impact should be limited.