According to Nintendo's Error Code Lookup, the 001-0502 error code may mean that some of the "ports" on the wireless router may need to be opened to allow your 3DS "to connect to the Friend List feature."
So it's possible that your Grandparents may have updated their wireless router (or it just automatically downloaded the firmware update) and slightly changed one of the settings so you can no longer now use it with your 3DS.
Now as to whether they or you're comfortable in trying to change the settings, you can check out the above url (and copy&pasted below) for more information, or they/you could call Nintendo's 1-800-255-3700 (6 am ~ 7 pm Pacific, everyday) Customer Service number, and one of the service representatives could talk you through the steps.
And if still no luck, the service representative can do some more over-the-phone troubleshooting to find out what else might be the cause and help correct it.
Situation:
- Unable to connect to the Friend List feature.
- This may occur while playing games online.
Most Common Cause:
This is most often the result of your wireless router or home network blocking traffic necessary for the Friend List feature. The following UDP ports may need to be forwarded within your router:
- 9103
- 50000-60000
What to Do:
- Prior to setting up port forwarding in your router, you must manually enter an IP address into the Nintendo 3DS Internet settings.[How to]
- Use your router settings to forward the ports 9103, and 50000 through 60000, to the IP address you provided your Nintendo 3DS system. [How to]
Situation Not Resolved:
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