In a classic case of skim reading the documentation we had trouble publishing a Skype for Business environment externally using a Microsoft Web Application Proxy (WAP). This was mainly impacting the Skype for Business mobile client which was failing to log on.
You could still access all of the standard web sites through a browser such as dialin and meet though.
This happened due to a missed step when setting up the WAP. If the internal and external URLs are different, you need to disable the translation of URLs in the request headers. Use the following powershell command on the WAP server.
$Rule = (Get-WebApplicationProxyApplication -Name "Insert Rule Name to Modify").ID Set-WebApplicationProxyApplication –ID $Rule –DisableTranslateUrlInRequestHeaders:$True
Once completed reload the mobile client and it should connect without issues.