Standby battery drain so fast

Hi, Im from Viet Nam, my redmi note 4 has problem with stanby battery, it drains so fast, maybe 15-16% in 6h, I found that my rmn4 has problem wih phone signal, if i turn on airplane mode, it doesn't drain that, other user in VN have the same problem too, can anyone helps me fix that? Thank you so much, sorry for my bad english

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