Lightbulb [Q] ZP700 3G Drop Signal when WCDMA is Active

Hi,

I bought 2 ZP700 that has MTK MT6582 processor.
In one of them when I try to used the WCDMA band I lost (drop) the signal. You can see the video:
youtube.com/watch?v=a6DPLloNMM8 ( I can't publish a link)

This is the phone information (works fine)
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And the other phone doesn't work
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The phone has problem to use the WCDMA band. In that case the band of my country is 850 Band that is supported for this device.

Can I change the base band ? What things can i do to fix the problem with the WCDMA Band ?

Thank for your help