Gateway IP address conflicts

It sounds like that gateway might have some problem possibly. Connect one client only and configure it with a static IP address, 192.168.12.10 with a 24 bit mask 255.255.255.0 and set the gateway IP to 192.168.12.1. Set DNS to 9.9.9.9 if you want or whatever even the gateway IP is fine. A client with a static IP assignment outside the DHCP scope should NOT see an IP address conflict. 

Open a console and ping the broadcast address 192.168.12.255.

Then issue the command, arp -a

See what ARP entries appear. With the single client I would only expect to see the gateway IP. Even if you do not provide the gateway IP address to the client it should be able to receive an ARP response from the gateway. 

That is a rather odd problem to see. IF you have tested with a single client maybe two and no additional switch connected and the problem is still there I would have to suspect there is something strange going on with that gateway. Maybe a reset to factory default and set it up again and then see if the same problem presents again. 

I confirmed using IP network monitoring software that the DHP is assigning all unique IP addresses on the network with no conflicts. Notably, the same network and switch never had this warning using a Verizon or Comcast router? Still getting the annoying "Windows has detected an IP address conflict” error message on every computer on the network every few minutes with that T mobile router.

The Arcadyan gateway with its wireless connections and the Ethernet ports are all part of the same Ethernet switch domain and the Netgear 16 port switch with a single connection to that does nothing but extend the switch domain for more interfaces. My suggestion is to temporarily disconnect the connection to the 16 port switch and only have a couple of wireless clients connecting with addressing allocated via DHCP. Look for the problem. If the problem does not exist then add and check as you go with other wireless clients and confirm you do not see the behavior. If you do not see the initial behavior connect the Netgear 16 port switch with a single Ethernet cable and see if the problem surfaces. If the Netgear 16 port switch is a managed switch and has the capability to run a DHCP server get into it and confirm that there is NO DHCP server running. Make sure no clients have bridging enabled. 

In effect add one thing at a time. When the problem presents itself carefully examine the configuration of the client that appears to trigger the behavior. DHCP servers can also be running on clients that have 3rd party software running. If there is a loop on the network that will cause frames/packets to spin. Make sure you do not have a loop. The Netgear switch may actually run spanning tree which should prevent any loop on the switch, if it is running. Divide and conquer. 

You could leave the setup as is and remove one device at a time and see if the problem stops. If you see the LEDs on the switch all lit up hard or flashing rapidly there could be a loop. If STP is not enabled on the switch and it is a managed switch then enable STP to prevent a loop. 

Thanks for all the suggestions regarding the "Windows has detected an IP address conflict" on all PCs on the LAN. I should have provided my LAN setup...

T mobile Arcadyan KVD21 Gateway Router (excellent 5G signal and speed).
Netgear 16 port switch (directly connected to router).
About 10 windows 7 or 10 computers connected to the Netgear switch via ethernet
Few T mobile cell phones intermittently connecting via 2.4 GH wifi.
No additional router on the local network. 
I wasn't aware the device allows for static IP addresses. A printer may have connected via a fixed IP address using the printer setup software, however.
All the windows computer devices showing windows IP network conflicts every five minutes.
Devices populate and seem to have unique IP addresses all in the allowed range as best I could tell by looking up each PC address. 
The conflicts did not occur with prior fiber and copper routers using the same network configuration.
Internet Protocol Version 4 (TCP/IPv4) is set to Obtain an IP address and DNS server address automatically on each PC.
Rebooting the router and switch fail to stop the conflicts.
Releasing and renewing IP Addresses on each PC failed to help.

T mobile support suggested the windows computer IP address conflict error relates to frequent transient loss of connectivity (? the timer function mentioned above) with conflicts arising from the Arcadyan KVD21 Gateway DHCP frequently reassigning addresses. She suggested splitting the networks (2.4 and 5 GHz) to minimize IP address conflicts when reassigned. I'm not confident this would work.

Sounds like you have two DHCP servers running. This is the service that automatically hands out IP addresses on your network. If you're using a router behind the T-Mobile gateway, put the router in Access Point mode, or simply turn off DHCP services on the router and let the T-Mobile gateway do the heavy lifting.

Are you only using the gateway and no additional router or switches on your network? See if you can get the T-Mobile home internet mobile application to work and connect and see what devices are reported on the network. I find I have to wait a little for the devices table to populate before I can see any devices. It is probably a timer function so the table of devices does not populate right off.