@kurttt Yeah that's what I meant. When you connect Frontier modem/router to main Deco, do you put your Frontier router in bridge mode (or passthrough mode)? Or do you have all Deco units running in Access Point mode? The reason why I ask that is, if you cannot get rid of your ISP modem/router, then you should put your ISP modem/router in bridge mode (means it only pass through internet to your next device, which is the main Deco, and let your Deco run in router mode to handle all DHCP requests, routing traffic etc). If you don't want to do this, then you should put all Deco in Access Point Mode (in Deco app, it should be in one of the settings). But this will remove meshing features like roaming capibility or some other features (QoS or sth I don't remember).

Try that, to see if that fixes your issues. Otherwise, I suggest to double check the connection between your main Deco and your satellite Deco, any obstructions in between, thick walls etc (if you can, wiring your Deco units is better).