@CalvinHobbes,

So a few things:

Because you bought this in a pack, the units should have attempted to connect to the network automatically without a reset.

Next, the Blue Blinking Light simply means that it was ready for the setup process.

Since the light has changed to RED, meaning that the Deco is setup and is now disconnected from the network, I would recommend reconnecting the Deco via Ethernet, and making sure that the wireless networking and backhaul options are all enabled. 

Since you also have three of the same unit, an easy solution with no troubleshooting would be to swap the satellite deco and the main deco units.

  @CalvinHobbes,

Given how many different arrangements you went through, it sounds like there may be a problem with the wireless backhaul feature on the last unit. If I remember correctly, the Deco units only check for which backhaul method to use once per day and when the boot up. I would try to reconnect the unit with the ethernet unplugged to see if this will allow the unit to choose a wireless backhaul.

What do you mean that you were unable to input the PPPoE settings? This is the first screen that should appear when going through the setup process. Was your phone never able to detect the unit?

I would recommend also getting in direct contact with our support team and pointing them to this thread, as they may be able to better know if this is a fault or setting issue.

TP-Link Support Page

  @CalvinHobbes,

You cannot choose between 2.4Ghz and 5Ghz because both your device and router will negotiate and trade between the two bands as needed. That way you will always be connected to the strongest available band. 

For channel selection, manually selecting channels in not possible for our Deco Units, as there are many factors that can affect the performance of a channel in a mesh network that may not be easy to find. The Deco Network,  whenever it optimizes, will select the channels that have the least traffic and interference automatically