@Sumit Gaur Thanks for reaching out. Based on the information you provided, it seems that the issue is related to SSL certificate validation when SAP PI is trying to make a call to your APIM endpoint.
You had obtained the SSL certificate for your APIM endpoint and installed the certificate in your system. However, when you try to make a call to the endpoint over HTTPS, you are getting a TCP connection reset error and the connection is failing over an SSL handshake, correct me if I am wrong here.
To establish a secure connection between SAP PI and your APIM endpoint over HTTPS, the SAP PI system needs to trust the SSL certificate presented by your APIM endpoint. You can obtain the SSL certificate for your APIM endpoint by browsing the endpoint and installing the certificate on your SAP PI system. This will allow the SAP PI system to trust the certificate and establish a secure connection with your APIM endpoint or It's also possible that there is a firewall or network security group (NSG) setting that is blocking the connection.
You can browse to APIM endpoint using Chrome and once there, click the icon next to the url -- Connection is secure -- Certificate is valid -- Details tab -- Export button.
do let me know incase of further queries, I would be happy to assist you.