How can I solve the network problem

I would ensure The antenna contacts on the mother and daughter board are making good contact with the plastic logic board board covers as they contain copper antenna traces for lte wifi and bulethooth.

I'd also check the coaxial cables that run along the side of the battery compartment, they are very prone to damage during repair.

The galaxy a12 often. Has problems with dust dirt and liquid ingress onto the lower charging sub board so there is a chance it may need replaced after 2 or so years of usage.