Check -> iPhone type & iOS version -> Go to Settings -> General -> About.
If it is an iPhone 6 Plus is an outdated iPhone (9 september 2014), > 7 years old, which also affects the battery life. That may be the cause of the problem. It is no longer possible to have the battery replaced at Apple.
If it is an iPhone 6S Plus see here: iPhone Battery Replacement - Official Apple Support
Having the battery replaced by Apple or an Apple Authorized Service Providers may resolve the issue.
If the problem persists, you can 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.
See: Contact Apple for support and service
Contact Apple for support and service - Apple Support