PBS Passport [1] is what you are looking for if you want to watch content that's outside of the original availability window.
[1] https://help.pbs.org/support/solutions/articles/12000043556-...
[1] https://help.pbs.org/support/solutions/articles/12000043556-...
https://bento.cdn.pbs.org/hostedbento-prod/filer_public/PBS_...