Accessing my account

Hi! I recently learnt that my Grandparents set up Premium bonds for myself and my two siblings. The only thing is, I have no idea how to access them. They put in £100, not much, but I assume that it will be more now! When I go on the website, there is a few options. I don’t know if we have actual online accounts, so I used the ‘Tracing Service’. That was a good few months ago and I haven’t heard back… My poor grandparents don’t know how to access our accounts either. Do I need to mail in the registration form?? Is there not an easier way to do it? It seems ever so complicated 😖