It is not 100% clear to me what you mean by speaking about the access through untrusted domain error. Are you sure that it is shown in the web browser because the certificate cannot be validated or because you haven’t set the trusted_domains parameter of Nextcloud?
If you meant the following message, you should read e.g. this posting: Nextcloud useless due to "untrusted domain" error
Concerning "Is there a method of generating the SSL cert that doesn’t rely on port 80? " you are also speaking in miracles. Do you want to generate a Let’s Encrypt certificate or what do you mean?
If you’re speaking about Let’s Encrypt, it might be of interest for you that, beside http-01 (80/tcp) the protocol tls-alpn-01 (443/tcp) is also supported. See Deploying Let’s Encrypt certificates using tls-alpn-01 (https) | by Sam Decrock | Medium for more details.