I am getting 'Service Not Available - 503'. But when I try with postman, it working fine. I have also update remote site access as well. Please specify if some setting needed regarding IP or something. I from Asian region, input if there are server setting needed for IP ? need to add salesforce IP to server or register IP ? anything like that ?
Please refer this help doc for IP Ranges and to understand best practices and why we need to do it. https://help.salesforce.com/articleView?id=000003652&type=1&sfdcIFrameOrigin=null