I have the same NVR
and use it via hik-connect.
Select Configuration, Network, Advanced Settings, Platform Access, enable it, i've selected HikConnect and then within hikconnect, login and select the device and enter the stream encryption code, so you'll see the streams.
I didn't set that up, as hikconnect could connect to my nvr directly, so in my case no special things are forwarded. but that can off course be different in your situation or isp requirements. I guess you experience it right away if you install hikconnect on your mobile and try to access your device (not via wifi).
On the NVR if you go to Configuration > Network > Basic Settings > TCP/IP, what DNS do you have set?
If the DNS is blank this may be what is stopping the NVR from going online, you can fill these empty boxes with the Google default DNS --- Preferred: 8.8.8.8 / Alternate: 8.8.4.4
There shouldn't be any other steps before enabling Platform Access, this is our guide for enabling Hik-Connect and it doesn't mention any steps before enabling platform access.