I can understand what you are trying to say. Contact TP-LINK support team directly. http://ticket.tp-link.com/index.php?/Tickets/Submit

The dlink cable is in bad quality and you'd better buy another one.

If you have confirmed what I said, submit a ticket here. Maybe support team can remotely access to the web interface and help you to look into it. http://ticket.tp-link.com/index.php?/Tickets/Submit

Where did you place the WA850RE? How many signal lights on it? You have ensure there are 3 at least. If not, you have to move it closer to the router. What's the firmware version of your WA850RE? Try...

Are you connecting to the router by cable or wirelessly to download? Does it happen different clients also?

Well, you found out the answer by yourself. The Unifi firmware doesn't support guest network feature while the latest firmware with guest network feature doesn't work for unifi. I would suggest to...