Thanks for your feedback.
We just checked the registration link, and it works well.
To use the link, please make sure that the customer is a new user.
The link doesn’t work with the existing user.
If the customer has already logged in the https://issuhub.com, the customer should sign out first, then use a new email address to create a new account through the link.
Or ask the customer contact us, we can give the customer a coupon code to activate the pro account.