I stand corrected a bit.
It is possible that yesterday before I noticed that the guest wifi has reappeared, I had switched the access point B on and off while I was messing around with the wire cabling - I had forgotten my Powerline actually.
So this morning, with still no sign of the dlink-guess, I switched by accees point B off and on. The guest wifi came out right away. NOK.
I switched it off and on again, and I could not see it. NOK
I again switched it off and on, and I could still see it. NOK.
I remove the backhaul from the port 2, power it off and on, and I could NOT see it. OK this time.
I put back the backhaul in the port 2, power it off and on, and I could see it. NOK.
I put back the backhaul in the port 1, power it off and on, and I could see it. NOK.
I removed the backhaul, power it off and on, plugged the backhaul after the AP had power on, and I still could NOT see it. OK so far.
To make a long story short, the guest network may came out, or not, right way after a power switch. This seems to be conditioned by the backhaul wiring at boot time.