@rhodester,
There are some features that can interrupt a device's connection if they are not compatible. Typically, you will want to look for settings in your router/tether app related to fast roaming and beamforming. You should also consider checking or lowering the encryption method used by your network - many older or IoT devices will prefer an older method such as WPA2.
You may consider disabling the Smart Connect feature and configuring the network to use two seperate networks with the same network name. This can help with devices that will not connect to a mixed network. Also, make sure that your channel width is set to Auto or try using a lesser channel width to test.
Was this the same network name that oyu had previously? If so, you may need to forget the settings related to your older network on devices such as your Kindle.
For your Sengled Hub, make sure it is searching the network and not via Bluetooth or a Direct Phone-Hub Wi-Fi Connection. If it is using your network to find the hub, try disabling your cellular network in case the "search" is happening across the wrong data connection.
You could also consider adding these devices to your Guest or IoT network if your model supports it. These networks often are meant to be compatible with more devices and may provide a clue.