Learn how to unbrick/flash this device using sp flash tool from here
Note
Follow the instructions on how to flash the device carefully from the link above
I will not take responsibility on whatever may happen to your device.
Make sure you are downloading the exact ROM of the device you want to flash because
flashing a wrong preloder, will make the device completely dead
(can't power on, can't charge and will not be detected by a computer)
SPECS
Platform: Android 4.4.2 OS (KitKat)
Dimensions: 118 x 63 x 12.9 mm
Display: 5.5-inch IPS Touch Display 720 x 1280 pixels, 267 ppiProcessor: 1.4GHz Octa-core Processor
MediaTek MT6592
Mali 450 GPU
MediaTek MT6592
Mali 450 GPU
Storage: 16GB ROM up to 32GB microSD card
Memory: 1GB or 2GB RAM
Rear Camera: 8 MP Camera with LED Flash, AutofocusFront Camera: 2 MP Secondary Camera
Dual SIM
GSM/EDGE 900/1800MHz
WCDMA 2100MHz
3.75G Internet (Up to 21 Mbps)
4000 mAh Li-ion Battery Capacity
INFINIX X551 (16+1) STOCK ROM
Android 4.4.2 Kitkat
DOWNLOAD
INFINIX X551 (16+1) WITH XUI INTERFACE
Android 5.1 Lollipop
X551-A1-L-20160325
X551-A1-L-20160115
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INFINIX X551 (16+2) STOCK ROM
Android 4.4 KitKat
DOWNLOAD
INFINIX X551 (16+2) WITH XUI INTERFACE
Android 5.1 lollipop
X551-BC-L-20160325
INFINIX X551 (16+1) CWM BACKUP ROM
Learn how to unbrick/flash device with CWM from here