{{UPDATE:
I figured out the problem. I had the NGINX file set on the “external” IP:8501 and not the Network IP:8501, equivalent to localhost:8501. This issue was figured out.
So make sure you use the http://localhost:8501 - replacing localhost with the IP it displays when you run it in the tmux session.
Note that this setup has proven to work great - so anyone setup to run their app under an SSL subdomain, this has been working fine, sans my dummy mistake explained in great detail here.}}