You need to configure the Global Permissions to grant anonymous access, but this will also make your content public (unless you are using with JIRA Service Desk). See https://confluence.atlassian.com/confcloud/global-permissions-overview-724765488.html
JIRA and Confluence share user management, so although they are different applications there is only one "instance". If you're logged into JIRA you will not be anonymous because you are logged into the "instance".
To allow JIRA users to access the same spaces as Anonymous users, give the JIRA users permission to access Confluence, then set up a group in User Management and add the JIRA users to it. Then give that group the same View permissions as your Anonymous users. Make sure you remove your JIRA users from the default confluence-users group.
The one drawback to this is that each of the JIRA users will be counted against your licence cost for Confluence. If you're not close to using your licence allocation for the tier you're in, this won't cost you anything extra, but keep an eye on your user count to make sure you don't get automatically moved into the next tier (unless you're fine with moving to the next tier, of course!).
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