It's not just you (App and Website both broken - support, absent)

[Crystalized Butane]

The website has authentication issues; bad HTML. That should have told
me something, but I kept trying.

The app rarely starts. It must be started in order to login to one's
own site, which one cannot do if one cannot log into the Serversman
site. Even when the Android app does start, one can get only as far as
the second login screen where the remote-host (phone) operating system
is selected by the user. But it (the Android logo thing) is always
grayed out and the little "close" link doesn't do anything, either.

There are no docs anywhere to be found to explain how to open the
required ports - not even a pointer link to such docs.

I would not trust the app nor the host service provider if they cannot
get any of these things right, at all. The system requires some level
of trust.