You do have an SSL certificate with your host and that’s exactly what expired and what you need to fix.
The screenshot you posted is not relevant here as that is for client authentication.
Again, talk to your host to get your certificate fixed, then it will work. You can also use an Origin certificate on your server of course, but it would be easier if your host simply fixed their certificate.
These instructions are unfortunately misleading as they recommend an insecure legacy setting. If you do not choose Full Strict, you have no security in the first place.