1. Check the signal strength on the phone screen and move to an area with better reception.
  2. Set your phone to use 2G (GSM) network only (Settings > mobile network (or cellular) > highest connection speed) in case the operator does not permit you to use 3G or 4G service.
  3. Reset your active network connection by switching Flight (airplane) mode On and back Off (Settings > flight mode). 
  4. Restart your phone by pressing and holding the Power and Volume Down keys simultaneously until the phone vibrates. Release and the phone will restart. After this operation, you may need to re-adjust the volume and set date+time on the phone.
  5. Check that your SIM card is fully inserted. Do not use SIM cards that have been modified to fit the SIM slot.
  6. If possible, try your SIM card in another phone.

If you continuously experience low signal strength in a certain area, network coverage may be insufficient there. For more info, contact your network service provider.