@Tony 1. One is a Volvo S60 vehicle, the others are a HP laptop and an iPhone X. The latter two can connect to the main SSID without problems, but need then connected to Guest... just cannot see them...

How can we see a list of devices connected to the "Guest SSID"? Deco S4 is deployed in Router mode with "Main Deco" linked (Cat5e) directly to the Cable Modem and the other port wired to a TpLink...