Free Flash Drivers

Hello everyone,
Before start, i want to tell you im French, ill do my best for having a good english.

Here is my problem :
Ive a Samsung Galaxy S6 SM-G920F, root.
Everytime im flashing a new ROM, i always have the same problem : Not registered on network
So i cant phone / sens msg / have 3G etc ...
I usually RESTORE my EFS and RADIO with recovery and my problem was solve.

BUt first question : 1/ Why i always have to do this ????

Here is the new problem, i recently flashed a rom version 4DPG2, everything went good BUT i still have my bug not registered on network. So i restore EFS and RADIO and this is not working cuz u cant downgrade a MODEM 4DPG2 in 3DPG.
I tried to only resto my EFS but it changed nothing.
For information, my IMEI is still present

Question 2/ How can i solve my network problem ???

Thank you for your help.