Nexus 5 was introduced in two versions.
1. D820 (For US Sprint and AT&T) only
2. D821 (For rest of world)
D820 supports all 3G frequencies but different LTE bands like band 2,4, 6 etc.
D821 supports all 3G frequencies and again different LTE bands like 3,5,7 etc.
Now as Google does not officially sale there nexus series in Pakistan so most of these we got are smuggled from US.
US ones D820 don't support LTE band 3 1800mhz which is supported by WARID and ZONG. Because hardware modems and antenna configuration is different. But at developers at XDA were able to enable band 3 1800mhz on D820 and claim the hardware supports it. I have seen people there successfully able to connect to LTE band 3 on D820. I will myself be going for hack as soon as ZONG or WARID launch there services within my vicinity.
So if Warid or Zong have enabled there services in your city, my frank suggestion is to try it, and it should work as D820 hardware does support LTE band 3 1800mhz.