I have a Deco x60 system with three Deco's. I initially installed two Deco's, one attached to my modem and the other as a satellite node. I immediately updated the firmware on these two Deco's to 1.5.1 Build 20210204 Rel. 50164. After installing this I noticed a few problems:

1. If my device is connected to the Deco that is connected to the modem (the main Deco), the internet speeds are very good; close to the maximum speed for my plan with my ISP, approximately 500 Mbps. However when I am close to the satellite Deco, my speeds are significantly slower, approximately 10-20 Mbps. The satellite Deco is less that 20 feet away. I have also tested the satellite Deco by placing it next to the main Deco; the speed is still slow.
2. On the satellite Deco, the Source Signal will only show "2.4GHz" and it will never show "5GHz" or "2.4GHz/5Ghz." It will only show 2.4GHz regardless of its distance from the main Deco. I have tested this by placing the two Deco's next to each other. 

I know that the maximum speed for 2.4GHz is 150 Mbps so this would explain why I will not get the maximum speed for my plan. But 10-20 Mbps seems like something is wrong. 

I decided to install the third Deco on my network without updating the firmware. The firmware for the third Deco is 1.0.5 Build 20201013 Rel. 64247. After connecting the third Deco to my network, I noticed that the speeds are just as good as the main Deco (good news). Furthermore, on the mobile app, the third Deco's Signal Source shows "2.4GHz/5GHz." I believe the newer firmware 1.5.1 is causing satellite Deco's to only accept 2.4Ghz signals from the main Deco.

Is this something that others are experiencing?

If it is the case that the newer firmware is preventing the 5Ghz from being used on satellite Deco's, can I downgrade the firmware to an older version? How do I perform this downgrade?