When installing your lock, you also configure the bridge to use your Wi-Fi network. However, if you replace your router or change the Wi-Fi network name, the bridge will go offline because it doesn't have the new information to log into your Wi-Fi network.
The easiest way to get your bridge online again after replacing your router is to change the Wi-Fi network name and password back to the one your previous router used.
You can find instructions to do this in the manual of your router or your internet provider's website.
If it's impossible to change this, or you would like to use a new password or Wi-Fi network name, it is essential to reconfigure the lock. Please follow this manual on how to do this: How do I reconfigure the lock?