Hi,
Based on my understanding , teh laptops connect to domain through VPN, and when the password near to expire , you want the users change it , but some users can't do this due to the error :domain unavailable .
Try the following way provided by Candy:
https://learn.microsoft.com/en-us/answers/questions/84842/how-to-change-domains-users-passwords-through-vpn.html
If there are any updates for the issue, please feel free to let us know.
Best Regards,