What to run
Use Is This Website Down, then SSL Expiry Checker, then Port Check. All three results are from Cloudflare.
How to read the result
Down-check on https:// tells you the URL responded.
SSL Expiry Checker tells you issuer, not-after, and days left.
Port Check on 443 only tells you a TCP handshake.
What to open next
Run them in that order.
A live 443 with a near expiry is a certificate job, not an outage.