Wifi and other disgusting issues

I have my m2 note for no more than one month, but I find this phone is very very very veeeeery troublesome. Since I use this device, the wifi signal has never shows “full” anymore. At the beginning I thought it was the problem of my router, however, when I take it to school, the wifi issue still exist. When I update the system to 4.5.3, the situation become even worse! I cannot call anybody at all!!! The dialing app crashes as I try to open it! Now I have no idea whether I should consider it as a “phone”!
Could anybody help me solve this problem?

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