If you can read this, DNS, routing, the port and the TLS certificate are
all working from this machine.
Sideload
Download manifest.local.xml
— already rendered against
.
Excel on the web: Insert → Add-ins → Upload My
Add-in → pick the file you just downloaded.
Excel for Windows: save it to a shared folder, add that
folder's UNC path under File → Options → Trust Center →
Trust Center Settings → Trusted Add-in Catalogs, tick
Show in Menu, restart Excel, then Insert → My Add-ins →
Shared Folder.
The task pane itself is at
/taskpane.html.
Opening it directly in a browser is expected to sit at
“Loading Office…” forever —
Office.onReady only fires
inside Excel. That is not a failure.