That’s to be expected--zerossl.com doesn’t issue certificates; they’re simply a web-based front-end to Let’s Encrypt.
I don’t know what you uploaded or where, but you’re still serving an old cert–it expired on 17 April. You’ve issued five more certs since then (see here for details), most recently today, but your site is still using the cert issued 17 January (which expired 17 April). This is just one of the many reasons why you should use an automated system to get certs from Let’s Encrypt, and that’s really what you should do here. This looks like a good place to start: