This is how to add yourself as an Installer to the child org you’ll be working on using Namespace.

Starting from the Parent Org, go to Users > Users, open the filter on the right-hand side, toggle Namespace, and copy the namespace.

Select the Child Org from the top right corner that you will be working on.

From the Child Org that you'll be working on, go to Users > Users and click the plus (+) on the top right of the page to add a new user.

1. Enter your email and check the 'Add a user from an existing namespace' box.

2. Add the namespace you copied from the parent org.

3. Toggle Portal Access and give the Super Admin role.
   

4. Click Save and proceed to the Credentials and the Access tabs giving yourself credentials and access permissions.