YouMail needs cellular data to successfully dial the activation code(s)
Since the activation codes attempt to add a forwarding number in your carrier's "Conditional Call Forwarding" setting, a strong connection is needed to make these changes. Some common error codes with poor connection are: OCN and MMI.
Wi-Fi calling, while it enhances your connection, does not supplement for the issues of poor cellular connection. Using Wi-Fi for the activation codes will result in the same errors, as cellular data is needed to make changes in your carrier settings.