Please check that you are under 3G area or not by using this link: https://www.grameenphone.com/personal/internet/see-also/coverage-map
Then make sure that you are using 3G enabled device.
Please configure your device by using this 3G settings properly: Network Mode as Dual or WCDMA/3G/HSPA/HSDP/HSPA+/UMTS and Uncheck 2G network only. Then switch off and then on your device for several times.
After that, hope you can use our 3G service properly