Your software
My Mautic version is: 3.3
Is there a way to view contacts and anonymous contacts visiting a specific external page?
yes, you can build that report in the reporting module.
Your data source would be page hits, under columns choose the data you want to report on. In your case include ID so you include the anonymous contacts.
For filter choose hit url and then enter the url.
Of course you need to have the Mautic tracking url set up on the external page.
I have not tested but this should work.