I have an existing P9 network consisting of three units, a principal one and two satellites.  The principal one is connected by ethernet to my new fiber optic modem/router.  A second one is also connected by ethernet to the main modem/router via an ethernet switch.  At the moment, there is only one node that is operating wirelessly.  The existing network works fine.

I bought another pack of 3 P9 units in the hope that I could simply add these to the existing network.  The mobile app that I have downloaded for my iPhone does not seem to allow this.  I have also tried connecting two of the units by ethernet cable to the main unit in the existing P9 network.  In every case, I get a blinking blue connection, but my only option at that point, using the recommended mobile app, is to create a new network with the name of the new P9 unit, which I wanted to be a satellite unit for the existing network.  I have often been advised by TP-link's software to do something in the mobile app that is not present in the app, like press the "+" button to add a new unit to the existing network.  

On the one hand, I am inclined to return the new package of three units, it does not seem to be designed for use as satellites in a preexisting network.  I also have the problem that the app doesn't seem to recognize my existing network, and the name and password of the existing network were both changed a few years ago.  I cannot get into the web application to manage the existing network.  On the other hand, it seems very surprising that I am meeting all these roadblocks, various things I read and saw led me to believe that this was going to be easy, because all of the units are the same mesh model, P9, but it is anything but that.  

Any thoughts would be appreciated.