Defective galaxy s6 edge,network issue.

[AC Question]

I was gifted a defective s6 edge sm g925f.The problem is the network keeps on turning off randomly and mostly happens just after I click on the dialer app and I can't make a call for 1-2 mins till it recovers,but once I am in a call the network is very stable never disconnects (i tried even a long 20 min call and it was stable within the call).A friend told to change the motherboard but it's from another country so not in warranty and I have to bear the costs.i tried all stuff like switching network mode, factory reset clearing the cache,all that's left is to change the rom.Is it a software problem or a defective hardware.