Your network at home has a router and that router hands out IP addresses within a defined range, called a subnet. It’s almost certainly on the 192.168.x.x subnetting range. And it does this dynamically (using DHCP). So that’s the ‘big’ network in your home. And your ‘big’ network has an interface to the internet - the WAN interface. And almost all routers do something called Network Address Translation (NAT) to send messages to and from the devices on your 192.168.x.x network to the internet through the WAN interface. One WAN IP address becomes up to 254 IP addresses on your home network. I think you know all of this already.
When you start the NVR, and connect the cameras the network port you use to connect the NVR to your home network is the NVRs WAN interface. And the NVR gives each camera a fixed IP address within another completely different subnet. Usually the 10.1.x.x subnet. So your ‘big’ home network can’t see the cameras, just the NVR. And the NVR isn’t a router. It just knows 4, 8, 16 or 32 IP addresses it can use for a camera stream. And it doesn’t do NAT so the cameras are invisible to your home network, you only see the NVR on your ‘big’ network.
So when you plugged the DisplayStation into the NVR it powered up fine because it’s PoE but it was allocated an IP address range on a private network that has no between-channels routing capability so your DisplayStation could not see any cameras even though they were in the same subnet. Had you known the individual 10.1.x.x IP addresses of each camera you could have entered those individually in the IPC tab under devices and it would have worked because you don’t need a router to view a fixed IP address on the same subnet.
When you plugged the DisplayStation and doorbell into the PoE switch, the switch let those devices see each other and they worked fine. Again, not routed, but switched, with IP addresses handed out on the same subnet.
So, what you need to do is plug the PoE switch into your home ‘big’ router and plug the DisplayStation and Doorbell into that for power and access to the internet.
Then, under the devices tab on the DisplayStation, select IPC and give it the 192.168.x.x IP address of the NVR. You will then get access to the NVRs interface to view any camera connected to the NVR.
To record from the doorbell camera, manually add the doorbell 192.168.x.x IP address to the NVRs camera list and set to Dahua or ONVIF and it will record like any other camera stream. The doorbell will count as one of your camera licences on the NVR so if you have an 8 channel recorder you may only be able to plug in 7 cameras to one of the 8 PoE ports as you’re using the other licence on the 192.168.x.x interface.
I hope that clarifies it for you without talking down at you. Very often I find enthusiasts have great knowledge but miss little bits here and there and I don’t want to insult you or seem condescending.
Subnetting (and understanding what a router does) is the key to 99.9% of all networking issues in my experience.