When you face the problem not network found in Samsung S6, don’t worry apply reset method
but save your important data before reset the phone. After the reset your problem will be solved.
If problem not solve then use Second method, just open the phone and replace the highlighted components
in the upper picture solution.But try one by one this solution, first replace tcx200 if problem remains then replace
2nd one name u3000 ic .
I hope your signal problem will be solved.
Warning: if you are not professional mobile technician then avoid to do this work.