iPad can't seem to view sites / pages that include the port in the URL?
You might want to exclude 'www' if possible. This can cause the page not being available as www mainly is pointing to port 80. If that doesn't work it might not be an Apple issue, and SuperUser might be a better place to ask the question.

Or you could set a redirect on a url pointing it to the right location with matching portnumber.

I can run different sites with portnumbers in it, as stated in the comments above.
Example: Synology NAS port 5000, works on my iPad.