iPhone 6s Plus bottom portion of the screen not working

If the problem persists, visit an Apple Store or an Apple Authorized Reseller in your region for assistance to diagnose the problem.

And they could offer a possible solution.

You will first have to make an appointment due to the pandemic.

Call Apple in your region see: Contact Apple for support and service

Contact Apple for support and service - Apple Support

AhadMohammed wrote: "Yes it still has its original battery, maximum capacity is 82%"

82% is not healthy and is to be expected in a 6½ year-old iPhone 6s Plus. Battery weakness is almost certain to be the explanation for the behaviour you're seeing – so have the battery replaced and all will be well:

iPhone Battery & Power Repair

About genuine iPhone batteries

(Articles found by searching Apple's Knowledge Base.)

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AhadMohammed ~ Does your iPhone 6s Plus still have its original battery?

Also, what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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