Default Poor reception of 3G network on I9152

Hello guys,
Am in Nigeria, Africa using MTN carrier
but my galaxy mega 5.8 duos never connects to 3G or HSDPA service, its always on EDGE and whenever I turn network to WCDMA only, the network goes completely off and sees no reception, when I check other devices like my blackberry or SG note 2, i get the 3G network.
Please what could be wrong, have read all through forums but didn't seem to find a good answer or solution.
Also whenever I tried to access the service mode through *#32489# it doesn't work


Will be awaiting your responses
Gracias