macbook pro 2017 works with wifi 5Ghz and not with 2.4Ghz?

I hit a problem with my wifi router; although I solved it, I do not understand what happens. Maybe somebody can give an explanation.

I have recently installed an Orange Livebox fibre optic modem (in France) which broadcasts WiFi in both 2.4Ghz and 5Ghz. The default set up is to use on SSID for both and the modem and the device figures out which one to use transparently.

This setup works well with a number of Apple devices: a MacBook Air, three ipads, two iphones, an a watch. However, my 2017 model macbook pro (running the latest Mojave) had problems, and it sometimes lost network access. I first though that there would be a DNS problem, so I have set up using Google's DNS servers (it did help in the past), but no difference.

Having monitored the connected devices on the modem's monitoring web service I realized that the problem comes when the Mac switches to the 2.4Ghz, while it is stable when relying on the 5Ghz. So I separated the two frequencies into two different SSID-s for the WiFi (thankfully the Livebox makes this possible), and I use the 5Ghz SSID for my MacBook Pro.

As I said, the problem is solved, but I do not understand what is happening and why. Anybody has a good idea?