I have two Deco X20 units, a new ONT/router from my ISP, and one CAT6 cable. When I check the main ISP router, I get around 100 Mbps. The main Deco X20 is connected to the ISP router using the CAT6 cable.  
However, when I connect the Deco units, my download and upload speeds drop to about 5 Mbps. I’ve tried both AP and Wi-Fi Router modes, but the speeds remain the same (as shown in the Deco app). I’ve already set 5GHz for my main devices and 2GHz for our smart devices. The main Deco X20 is connected to the ISP router using the CAT6 cable. 

I'm out of ideas on how to fix this.