This article explains how to connect an Echo Dot to a Wi-Fi network. It also explains what to do if your Echo Dot can't connect to Wi-Fi.
How to Connect AMAZON Echo & Echo Dot to a New WiFi Network or If You Changed Your WiFi Password
When you set up your Echo Dot, connecting to Wi-Fi is part of the process. After that, your Echo Dot will remember your Wi-Fi network information and automatically connect as long as the Wi-Fi network is available. If you change your network, you will need to reconnect your Echo Dot to Wi-Fi.
Here's how to connect an Echo Dot to Wi-Fi:
In the Status section, tap Wi-Fi Network.