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For your security, Cox digital identities come in two varieties. The Account Holder ID type has full access to all account information and billing data. The Authorized User ID type is a restricted-access digital identity, which means that (among other things) Authorized Users can't view billing statements online.
You can read more about User ID types by visiting Your Digital Identity, an article on our Learn website.
Did you establish your household's account? If not, you are probably an Authorized User. Let us know in comments if this resolves the issue for you.