There are many sources that tells you how to troubleshoot the issue, but what I found in one installation is that the problem was that Web services External fqdn was not published in internal zone of split DNS. Note that we are talking about WAC in cloud (Amazon), so this architecture is not using Reverse Proxy.
So the solution was to ensure that from internal frontend we can do
and it points to external IP, and that we can do from internal frontend (U turn)
telnet lyncwebserviceexternal.domain.com 443
Once it is ok you should observe in Frontend IIS log something like
GET /DataCollabWeb/wopi/files/5D-1-1CE946B access_token=AAMFEO8PHq4......
to ensure there is a connection to get a PPT file from meeting share