Lava Mobiles — I buy this lava z1 pro phone 1 yr ago and now days I m having full network but phone is not supporting to reach internet

Hello, i buy my lava z1pro phone last yr and at that time this phone was very awesome and before 10 days also phone is working smoothly but now my phone is showing 4g net sign but itwas not working and the company from where is buy my sim card message me that this phone is not supporting network but i am not understanding why this happen becoz earlier it was giving me super fast net connection so why not now???? And also they told me to upgrade my phone and then i checked for updates but my phone is giving me notification that the phone is updated already ???? So can you please solve my prblm ...