If the hub has been offline since before March of this year… then the most likely scenario is the security certificate expired. Here comes the fun part. You should follow the steps to block NTP port 123 udp as outlined in the following thread: https://community.smartthings.com/t/troubleshooting-smartthings-hub-2018-and-aeotec-hub/298332
remember to unblock the port after onboarding the hub in the app and wait awhile for the hub firmware to update. You can check that in the app or in the Advanced Web App (AWA) that I linked in my first post.