Mobile network needs to connect everytime

I am from Bangladesh using blackberry z10. I use the carrier named 'Teletalk 3G'.
The main problem is the mobile network does not respond after sometime. I have data services on. If I turn off the data services and mobile network first and then reconnect them it works just fine and after sometime it shows that it's connected but no response.
But if I restart the mobile network and the data services and then start mobile hotspot ... then I can use the internet just fine like now..

But it's not possible to start mobile hotspot everytime as it eats up all the battery charges. I think this has nothing to do with mobile network as it gets feed correctly..

How can I solve this problem so that I don't have to reconnect the mobile network everytime
Thanks

how old is the phone? have you tried the sim in another phone?