Having problem in mi3

Hi everyone,
This karunesh, I am using mi3 for almost one year but now it started giving me trouble. Few days back it started having problem in connecting to local carrier. Whenever it's upper part is covered or in some place like under the table or bed or any basement or conference hall it always showed no network... I decided went to service center and I went, then they just wiped all the data and gave back it to me in the condition in which I purchased it and said that the last update given to it wasn't not supporting, as there are some problem in last update try to update when next update will come and I also asked them to check it's volume rocker and power button and he cleaned it in front of me... Now it has been three days since I am using it and yesterday night my network went again... And I am also having a problem with wifi after I visited service center , wifi signal that it catches is very weak, earlier it was not like this... Hence, guys please tell me, what could be the problem regarding these issue.. Is this really a software issue or there is something wrong in my phone's hardware.. What could be the reason of weak wifi signal... Sometime it is almost gone earlier it never happened to me it just started after I visited service center....
Thanks in anticipation

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