need help for my g925f no imei and base band after downgrading

My samsung s6 g925f marshmallow has samsung account and I reset it using hard reset button,i downgrade my device to 5.1.1 because there’s no accessilibity to bypass the accounts …. but after downgrading my s6 there’s no imei and baseband it say’s unknow,i didnt back up the efs,,, can anyone know the solution!!!
Thanks in Advance mga Boss,,, I really appreaciate your help!!! Pls Pls Pls help me…;)I have big faith that you have a solution….Gsm fighting!!!;););):D:p