After you connect your Amazon Echo Dot to a Wi-Fi network, sometimes, the device may show “offline.” When you restart the device, it can work properly for some time but might turn offline again. In such a case, you need to perform more actions to bring the device online. Continue to read to know why is Amazon Echo Dot offline and the different solutions to resolve the issue.
Source: https://how-to-setup.com/why-is-amazon-echo-dot-offline