Accessing content on hard drive connected to router via Plex

I connected my external HD to my Xfinity router, but Plex isn’t able to access the content. I have Xfinity Advanced Security disabled. What else do I need to configure? Do I need to set up a port forward? I couldn’t find where to do this. If anyone has a similar configuration (HD connected to Xfinity router) and can include screenshots of their configuration that could be helpful.  I've read some places to go to https://internet.xfinity.com/network and click 'Advanced Settings' to configure the port forward, but I don't have 'Advanced Settings' as a option.  Xfinity speed test is my only option listed in addition to showing my device