Not sure if you should be using anything external without fully understanding DNS and how Web Servers talk to each other 
I would assume that srvtest02.domain.local is not known to anyone but your local network… DNS 101 you are going to need to specify your external domain/ip, something that you could access outside of the network via your mobile phone on 3G Connection via a browser for example.
You would need to setup an external domain, make sure that 443 the SSL Port can be accessed from the outside by adding an exception in your firewall, among other things.
If you were Office 365 located in England and you did ping srvtest02.domain.local it would yield… nothing. Hence the
The provided host name ‘srvtest02.domain.local’ could not be resolved.
The reason it works via POSTMAN is… because its running on your PC connected to the network with awareness of the srvtest02.domain.local
Impeccable timing by @Bart_Elia his recent post Multiple Applications and DNS Tricks - Experts' Corner - Epicor User Help Forum might show you some examples.