Your Eco Dot/Tap, HS100, and Smart Phone all need to be on the same WiFi network, SSID, WiFi Password and 2.4Ghz band. Also, the way you named the device might be causing an issue. In my basement I have a TP-Link Smart Switch "Basement Light", Ecobee Remote Sensor "Basement", and Logitech Harmony Hub "Basement Hub". When I would ask Alexa "What is the temperature in the basement", She would respond correctly. If I said "Turn on the Basement Light" She would respond by saying she does not see that device or basement does not support that. Once I changed the Ecobee Remote Sensor from "Basement" to "Basement Sensor" all three devices started responding correctly from Alexa.

Because my wife has lots of purchased music under her Amazon account, we added her to the Alexa devices. All she has to do is say "change accounts" and the device(s) go back to my account and the smart plug works perfectly. When she is ready to access her tunes again, "change accounts" takes her to her account.

It is simply a toggle between the two accounts. Mine runs the smart plug, hers gains her access to her music.

This is not quite accurate. I have a portion of my smart devices on the guest network (those with weaker security ... such as the TP-Link switches) in order to wall them off from the primary network. I can control them just fine from devices on the primary. I do know that Alexa can get a bit finicky if you change the name of devices after the fact. I get it telling me I have more than one device sharing the same name (I don't). In the case of one device ... I repeat the same name a second time and it works ... but never the first time.