NPM can also do URL proxying: you can reach sub.maindomain.com
at maindomain.com/whatever/
. Edit the proxy host definition for the main domain and look in the “custom locations” tab.
Alternatively, if you can control your browser’s settings, you can try using DNS over HTTPS which can let you bypass your company’s DNS. Try using https://dns.quad9.net/dns-query
for example as the DNS over HTTPS address (or whatever your browser can offer).
“Shared network folder” in Jellyfin doesn’t do what you think it does. 😛 I agree it’s rather confusing. It’s just a convenient link to a Windows share which you can open from the Jellyfin app if you want to browse the files and they happen to also be shared as a Windows share. It’s NOT where Jellyfin takes the files from.
Jellyfin can only index files accessible to it locally. Share the files from TrueNAS to the machine or container running Jellyfin, then point Jellyfin to the directory where you mounted the share. I recommend NFS rather than Samba for this purpose.