To successfully connect, make sure that the CA has not been changed.
Amazon Web Service
Amazon https://www.amazontrust.com/repository/
AWS IoT https://docs.aws.amazon.com/ja_jp/iot/latest/developerguide/server-authentication.html
AWS Service https://www.amazontrust.com/repository/
Microsoft Azure
Azure https://docs.microsoft.com/ja-jp/azure/iot-hub/iot-hub-x509ca-overview
Azure IoT https://docs.microsoft.com/ja-jp/azure/iot-hub/iot-hub-x509ca-overview
Azure Service https://docs.microsoft.com/ja-jp/azure/security/fundamentals/tls-certificate-changes
Google Cloud Platform
GCP https://pki.goog/repository/
GCP IoT https://cloud.google.com/iot/docs/how-tos/mqtt-bridge#using_a_long-term_mqtt_domain
GCP Service https://pki.goog/repository/