By default, back (previous) issues are not available to a reader. But you as the publisher can choose to make your back issues available through two ways.
1) You can choose to make all back issues available to subscribers. Log into your TypeEngine account, and check the box under Publication>General> "Include all back issues with paid subscription"
If the box is checked, anyone with a subscription will be able to view all back issues. If the box is not checked, subscribers will only get access to the issue that is current when they subscribe and forward, and only as long as their subscription is active.
2) You can choose to offer back issues individually for sale. Log into your TypeEngine account and click on the desired Issue and then the Pricing tab to view this option. See this article for instructions.