Here are some basic NDI troubleshooting steps that NewTek Support will typically use for basic NDI setups:
1. Check that the Windows Firewall on the units is set to off. You can see how to do that here; https://www.lifewire.com/how-to-disable-the-windows-firewall-2624505
2. Ensure that User Account Control is set to Never Notify. You can see how to do that here; https://quickbooks.intuit.com/learn-support/en-us/install-or-update-products/change-user-account-control-uac-in-windows/00/201380
3. Go to your network settings and see if the network type is set to public. NDI will not work on a public network. You can make that change through the following; https://www.online-tech-tips.com/windows-10/change-from-public-to-private-network-in-windows-7-8-10/. For Windows 10, you may need to use the group policy editor instead, as outlined here; https://support.newtek.com/hc/en-us/articles/360012299633-Changing-your-Network-Location-Type-using-Group-Policy-Editor?source=search
4. Disable any antivirus software running on the machine.
5. Make sure that all the devices you plan to send and receive NDI amongst each other are on the same networking subnet. Not only does this mean that the subnet masks need to match, but so do the first 3 octets of the IP addresses. For example, a device with an IP of 192.168.1.2 can see 192.168.1.3 but it won't be able to see 192.168.10.4.
6. That you've installed the NDI tools pack on the receiving computer, which you can get from here; https://ndi.tv/tools/
7. Ensure all devices and cameras are on current software.