Default TLS cert is automatically created and self-signed, so it's easy to get it right based on few basic inputs.
If you create one externally you have to get more inputs right (host name, signing, chaining) and your DNS must be correctly configured to resolve hostnames, so while the cert itself may be correct it can still cause problems. I'd look at the bycast and other top logs, there's probably something about DNS or hostnames that cannot be found.
I configured DNS for SG nodes and created a self-signed TLS certs for SG and it worked fine for me on several v11 versions.
I seem to recall that I also tried to use the same Github instructions for that and those did not work. I'd try more recent instructions (you don't have to use StorageGRID-specific parameters) or check the logs to see what problem or error the uploaded certificates create.
Like I said, don't use that guide, it's outdated and hasn't been updated for a while. There is nothing NetApp- or StorageGRID-specific in those TLS/SSL certs. Nobody uses MD5 these days, its days are over.
Use a recent generic "how to create self-signed SSL cert" procedure from Google search (StackOverflow or whatever), or Microsoft or Linux vendor documentation.