These are the correct steps for adding a new UDID to an iOS provisioning profile. It seems there has been a partial iOS re-sign service outage at the moment you tried to re-sign your app. We are sorry for the inconvenience caused. You can use http://status.telerik.com/
as a source of information regarding all Telerik Platform services status and general health.
Just for clarification, the re-sign form does not accept a .cer file. It accepts only exported certificate files .p12, but I am sure this was not the problem since there is a validation on the form.
Let me know should you have any other questions.
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