I can't back up the chat history on the WeChat server for 7 days as I don't see the restore and backup buttons on my iPhone 6.
The Backup button will appear after you have tapped on the option in the Settings> General. This is the only way to trigger the appearance of those buttons. This is the only way to trigger the Backup and Restore function on iOS enabled phones and device where WeChat can be installed.
You can also use Syncios software for the same purpose: http://www.syncios.com/blog/how-to-backup-and-restore-wechat-messages/