Hi @Donn_Pasiliao,
Welcome to the community forums! 
Is there something in the dedicated cluster configuration that I’m not considering
Dedicated clusters do have a higher maximum connection count but include internal connections between the node(s) and also other atlas monitoring or automation agents. However, I don’t believe this should reach 1200 as it generally only counts for approximately 30-60 connections (out of a maximum of 1500 for M10 tier clusters).
In comparison to the shared tier clusters, the internal atlas connections do not get counted as a part of the connection count. e.g. A newly created M0 with no external connections should be at 0 connections in the Atlas metrics where as an M10+ tier cluster with no external connections would normally be expected to be at 30-60 connection count.
When I remove all IP addresses from Network Access, the lowest it would go down to is about 1200.
Where are these connections coming from and how do I troubleshoot it?
Even after some time has passed after removing all IP addresses from your Network Access list, you can open a support chat/case to verify what connections are being made to the cluster if you have 0 IP’s in the access list. Additionally, you can try view and download the MongoDB logs to see if there are any particular IP’s that are connecting the most often.
Hope this helps,
Jason