Tried by now: I passed from 4G to 3G and still have connectivity and mobile data on SIM2, the same for 2G which I am using to send this reply. I have the sensation that is about network operator OR because my SIM1 is set to 2G (I go to test). Nope, SIM1 4G and SIM2 in 2G connected with mobile data and here I am writing.
Customisations: my ofono is allowed to load the qmi plug-in which by default is disabled + the network on the SIM2 is set manually