Issue with AWS Public API Gateway

I created Public API Gateway for testing. There is an client VM machine which has nginx server. that machine send request to API Gateway. I faced one weird issue as when client nginx machine send request to API gateway , it receives 503 error as response but at same time not able to see any log at API Gateway end through Cloudwatch. But when i directly send request to API Gateway URL from my laptop, it worked fine. If i need to troubleshoot this issue, what will be the steps? If API Gateway is public, is there any way to capture network traffic of API Gateway same like wireshark?