Mobile Network Error / Network Connection

Hi.

I am using Samsung E7 and recently screen got wrecked and I went to a nearer Service Centre and replaced it with a new one. After that incident, as it is a Dual Sim mobile I installed two SIM cards which are good providing service operators. Even then I can't get signals for both of them and very hardly I see the TOWER symbol in the notification bar and the rest of the Time I only see this symbol