Do you see any error codes in the app when you are not able to add it?
It is possible the welcome code is valid but the security certificate on the hub has expired. If the hub has not been active since March 2025… then that is most likely the issue. The error code may profile a clue. If that is the case, following the guide in the following thread: Troubleshooting SmartThings Hub 2018 and Aeotec Hub
LOL and the most obvious that I forgot. If the hub is still claimed on someone’s account, you would need to reset it first and then try adding it.
to reset:
Disconnect power cable from hub and remove any batteries.
Connect an Ethernet cable between hub and your network.
Press the reset slot and while continuing to press, insert the power cable.
Wait about 30 seconds until you see a solid yellow led on the front of the hub. Then release pressing the reset slot.
Try adding hub to app.
If that fails, following the troubleshooting linked above.