I finally resolved this issue. There are a couple of changes need to be aware of with iOS 14:
Upgrade TLS version to v1.2. You can use LetsEncrypt for testing purpose, 90 days lates version of TLS is issued for free. You can use this for production purpose as well.
Life time of the certificate needs to be 398 days or less.
samjem
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I'm facing the same issue, but it's working fine on diawi. Someone has suggested to upgrade TLS version of the server to v1.2. I'm trying that, keep you posted if there any luck. Let me know if you find a solution. Here is the link for TLS version upgrade suggestion: https://developer.apple.com/forums/thread/652391