I bought Nexus 5 around 2 months back from USA. I am currently in India using Vodafone network (3 G).
I am facing this real weird issue . I can login to google plus and Hangout chat when on wi fi mode. But when I switch to mobile data mode , Google plus cannot be connected as a result hangout chat also stops . Application gives an error "Could not connect to server".
I have tried clearing cache and data from Google search , play service , Google plus , hangouts and tried re-login without luck .
I would also double check to see what your carrier operates on and make sure the Nexus 5 variant you have is supported. That's likely the problem and the model Nexus 5 you have won't work. There are two Nexus 5 variants available: North America and everywhere else.
Since you know for sure goolge hangouts works on WiFi and not on cellular, i would check,
1 - make sure your data is working properly on cellular. Check the APN. You dont mention if data works with other apps or if it works and just NOT with google.
2 - If you do have valid data with other apps on 3g and google still doesnt work its possible its being blocked in your country or by your provider.