I just got a new OEM battery installed in my old 6S that’s been in a drawer for a couple years.
Who replaced the battery?
Have you had the battery replaced by Apple or an Apple Authorized Service Providers?
iPhone 6s, iOS 10
For sure check your iPhone model & iOS version -> Go to: Settings -> General -> About.
If your iPhone is an iPhone 6S you can update the iPhone to iOS 15.7.3, see -> iPhone models compatible with iOS 15.5 -> iPhone models compatible with iOS 15.5 – Apple Support (UK)
You need about 6 GB device storage and good Service & Wi-Fi.
If they are On, turn Off Hotspot & VPN.
Charge the battery > 70%.
See the following Apple articles:
Update your iPhone, iPad, or iPod touch
Update your iPhone, iPad, or iPod touch - Apple Support (IN)
If your iPhone, iPad or iPod touch won’t update
If your iPhone, iPad, or iPod touch won't update - Apple Support
You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC
Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)
If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder
Update your iPhone, iPad, or iPod touch with the Finder - Apple Support
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See the following Apple articles:
Connect to Wi-Fi on your iPhone, iPad, or iPod touch
Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support
If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
If your iPhone or iPad won't connect to a Wi-Fi network - Apple Support
If they are On, turn Off Hotspot & VPN.