SSL Error while connecting through cellular connection

We have developed app iOS app using Xamarin.

It's working well in my test devices. My client is running into an issue of SSL error when he is using cellular connection.

Same app is running perfectly fine when he is using Wifi connection.

I am using Modern HTTP client to make web connection to use native APIs.

• I have also tested app in IPV6 environment and it is also working fine.
• Settings of App transport security are okay that I have verified. [If error is here then it should come all time irrespective of connection type.]

Issue is at remote end so, I dont have too much opportunity to experiment with various settings, debug and see what is causing this issue. Having any clue or direction would be really helpful.

Note: Using third party online tools we have verified that our server is ATS compliant and using TLS 1.2

Stacktrace

System.Net.WebException: An SSL error has occurred and a secure connection to the server cannot be made. ---> Foundation.NSErrorException: Error Domain=NSURLErrorDomain Code=-1200 "An SSL error has occurred and a secure connection to the server cannot be made." UserInfo={NSErrorFailingURLStringKey=MYURL, NSLocalizedRecoverySuggestion=Would you like to connect to the server anyway?, _kCFStreamErrorDomainKey=3, 

_NSURLErrorFailingURLSessionTaskErrorKey=LocalDataTask <5C93BDAD-10CE-478B-9D72-5AA02AA84617>.<1>, _NSURLErrorRelatedURLSessionTaskErrorKey=(
    "LocalDataTask <5C93BDAD-10CE-478B-9D72-5AA02AA84617>.<1>"
), NSLocalizedDescription=An SSL error has occurred and a secure connection to the server cannot be made., NSErrorFailingURLKey=MYURL, NSUnderlyingError=0x282ea41e0 {Error Domain=kCFErrorDomainCFNetwork Code=-1200 "(null)" 

UserInfo={_kCFStreamPropertySSLClientCertificateState=0, _kCFNetworkCFStreamSSLErrorOriginalValue=-9816, _kCFStreamErrorDomainKey=3, _kCFStreamErrorCodeKey=-9816}}, _kCFStreamErrorCodeKey=-9816}

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