No, Custom DNS settings will not take effect immediately. Any existing sessions will continue to use existing DNS settings.
After an active session is disconnected, your Air SIM device will receive the new Custom DNS settings when it reconnects and creates a new session. This can typically be done by rebooting the device, disabling and re-enabling the Air SIM network adapter, or other method.
You can also disconnect the existing session from the User Console by setting the Air SIM status to Inactive and then resetting it to Active.