@Symph Do you actually need such a large range of IP addresses? I have observed some anomalies when setting the LAN IP Subnet Mask to something else than 255.255.255.0. I suggest setting the DHCP...

@decoinRI I understand you have another deco, configured as a main deco, let us call it deco6, connected via the wifi bridge to deco5. Do you have the possibility, just to test, to connect it via a...

@JoeV1 Each satellite decides automatically to connect either to the main Deco, or to another satellite. With some Deco, you can manually override that, and configure a preferred neighbour.

I would first try with the main on the intermediate floor.

@andye85 To diagnose the issue, I suggest exchanging the position of the two satellites. That will allow determining if the issue is caused by the environment or by the appliance.