If you have sync'd your iPad/Notion for iOS with iCloud, you should be able to log back into iCloud on your new device, enable iCloud support for Notion, and then have all of your files re-appear. To enable iCloud support, you will want to go into your Device Settings, scroll down to the Notion app, and enable iCloud Support.
If you have not been using iCloud, you can manage your files using iTunes on a desktop computer. Here is an article from Apple that explains this process.
https://support.apple.com/en-us/HT201301