Teams won't talk to Azure Bot Messaging endpoint after the bot is published

Before the teams bot is published, the teams bot works fine when I tested with "Preview your Teams App (F5)" in Visual Studio Code. During the test, I used an endpoint sit on AWS which is publicly available. Then I published the bot. After the bot is published and I tested the bot again, I can see occasional portal.azure.com pre-flight (OPTIONS) request logged against the service, but nothing else. The Teams App ID is 9f006466-514a-4f7e-9220-be25c9ef5d19. I checked my endpoint supports TLS 1.2 with encryption cipher TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384. What's the problem?