Hi Stephen,
Thank you very much for replying.
As suggested by you , I made changes in service and configured self-signed certificate in WS-Security Properties assertion by selecting it from gateway trust store. Even though API service is getting same error from back end.
As I checked with relevant team on issue and they have reverted with below message.
"The API Gateway does not add the wsa(Webservice Addressing) in the soapenv:Header."
Back end expecting soapenv:Header and sent by SOAP UI as,
<soapenv:Header xmlns:wsa="http://www.w3.org/2005/08/addressing">
Actual soapenv:Header sent by gateway,
<soapenv:Header>
Let me know where I have to do configuration which request will send AS IS expected by target system.
Thanks,
Suneel