@venkat I followed your steps with ufw disable as well as allow 80/tcp and it still failed. I tried telnet 52.148.136.139 80 and 8080, both failed. I then noticed the NSG rule Name has a yellow triangle. Once I added a rule with the Src Port ranges changed to Any *, there are no more yellow signs and the index.html is successfully displayed.

telnet 52.148.136.139 80 also works.