Indeed, explicitly enabling/disabling the wifi guest, from the 'router' configuration, made the trick for me.
Long story, I reverted back to 'router' mode using the iOS application, played a bit around in the menu using both the iOS app and the console. Since I was not 100% sure that both the nodes A+B were properly reconfigured while doing, I properly put the node A behind my main router, disconnected the wire from the node B, then made another factory reset from the console. Doing so I noticed that a dlink-guest was still being broadcasted, so something was probably not yet properly defined.
So from a clean 'router' configuration, from the console, I enabled the dlink guest wifi, gave it another name, checked it was there, that I could login from another device, then I disabled it from the console.
Then I changed the configuration back to 'Bridge', also power off/on the node B.
Then I could not see anymore the dlink guest broadcasted the node B.
Cant be sure this is the root cause, but it seems to work so far.