SSL Error while connecting through cellular connection - ios

Question

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}
Update
We have updated server instance and that has made this issue random instead of all time producing. Removed HTTP client but still issue is there. We are neither getting steps or situation to produce it all time nor fix.

Answer 1

"I am using Modern HTTP client to make web connection to use native APIs."
This is likely the cause of your problem. This was something that was true several years ago. However Xamarin HttpClient already uses native APIs (you can set that in the project properties), while Modern HTTP client hasn't been updated for years and likely has some problems by now.

Answer 2

Is the connection failing all the time or sometimes? If the latter, then the bottom could help.
I'm not sure if this is the case, might be worth checking. But, we also had a similar issue on some Cellular Providers. What happened was the SIM had no more data (hit the limit), so the Provider would initially allow connection but cut it in the middle. This would show as an SSL error.
In our case, we could not do anything about it since it was the Cellular Provider's specific implementation.