You need to turn OFF router mode on the C series modem and then setup the MK system. Try setting up the MK system with a web browser and PC connectafd to the MR.
Netgear has set up a community forum specifically for the NightHawk MESH products. Most of the people who watch that forum are more likely to have experience with NH MESH products and know how to work it better than those of us who follow this router forum. Might be more likely to find someone who has a solution if the question is posted there: https://community.netgear.com/t5/Nighthawk-Mesh-System/bd-p/en-home-nighthawk-mesh-system
You can connect the MK system to the C6900 without having to disable the C6900's router.
You can connect the MK system and just put it in access point mode. That leaves the C6900 managing the routing functions with the MK system acting as an access point. You'd just want to make sure the C6900 has a seperate ssid and the wireless channels are on different channels to prevent interference with the 2 systems.
The "model not supported" is in relation to the app. Some devices aren't supported in the app. (C6900 would be one) so simply use the browser when connecting to the C6900. You can also use the browser to setup the MK system as well.
