I don’t know what your exact configuration looks like, so I’m just throwing ideas out there, but is there a chance that your network isn’t reachable at all through your domain (and instead only through your external IP address)?
The port check tool that you linked didn’t allow me to enter my domain (it just autofilled to my external IP). When I used the port check tool at portchecker.co, I could enter any value that I wanted, and found that port 443 was open for both my external IP and my domain.
If the port checker says that port 443 is closed on your domain but open on your external IP, it may be worth checking that you’ve correctly set up DNS records for your domain.
This is what my DNS records page looks like on Dynu:
The only records that I created myself are the ones in the red box (i.e., the A record and the AAAA record with the hostname *.mydomain.tld, and the A record points to my external IP address). You need to make sure that you put a star before the domain if you want to redirect all subdomains of your domain to your network.
If none of this helps, it may also be good to know how exactly how you are trying to create your SSL certificate. Are you trying to create it directly through Nginx Proxy Manager like I did with Dynu or are you using a different tool?