If you can access the site via a web browser and ignore the "expired cert" warning
I can do this in Firefox 128.11.0 on desktop
then you can do that and still access the site before they get the certificate updated.
The Eternity native client on Android can't do this.
That being said, I would assume that the cert is also required to authenticate the instance for federated interchange of data, so this may also muck with lemmy.today's connectivity to other Threadiverse instances.
EDIT: Yeah, I'm pretty sure that the cert being expired killed connectivity to other instances, as I'm not seeing posts from elsewhere newer than about four hours as of this writing.