@Beebader 

To avoid disturbing less sophisticated devices, I would only use letters and digits in SSID names, and start the name with a letter.

If some devices do not connect to the Deco wifi network, I would look at:

- the bands (2.4, 5) offered by the Deco.  The "dummy" device is perhaps confused by 2 bands used by the same wifi network

- the "mesh technology" offered by the Deco device (settable for each device)

- see also the comments, in the deco app, about the "beamforming" and "fast roaming" options