Please do as following:
Navigate to your SharePoint Wiki page and click PAGE > Page Permissions.
Click "Stop Inheriting Permissions" and then click "Grant Permissions".
Under "Get a link", select the link type with "no sign-in required" and then click Copy.
A guest link will be created and be valid before expiration date. It allows external users to have ...
Set the permissions of the wiki to "Anyone". Instructions are here: