Why am I not receiving 3G/4G signal on my phone?

Hi, I own a Nexus 5. I recently began getting this problem where my phone did not get 3G/4G signal at all. If it does get it sometime, it would stay only for a few seconds and then turns to a zero network where the number of lines turns down to zero. Then the only option I have is to shift to 2G. I tried the method of *#*#4636#*#* but it came to no use at all.

Please help me in this. I have no clue what to do with this problem now.