how can I access OSX server with iPhone iOS 12

Some questions:

Assuming OS X Server is at your home:

Where is OS X server exactly?

Do you have access to it? eg: Can you physically see/touch it?

Is it switched on?

Did you install and configure it?

Is OS X Server and your iPhone connected to the same network (Wireless and/or Wired)?

nb: Your home network will be usually provided by your Broadband Router.

If OS X server is at your workplace

Has anything at work changed in terms of the network your iPhone connects to?

eg: Does your iPhone connect to a differently named network than before?

Do you know who installed/configured/setup OS X Server?

If you do can you ask them why you can't access files anymore?

In what way are you trying to access OS X Server? For example do you want Mail, Web, Contacts, Calendar services or simply a File? You have to be more specific.