Un able to install update

I had the same problems on my mother's iPhone 6S

Installing the update via a pc/Mac helped my solve this issue

See this link if you're want to update from a Windows PC or a Mac with Mojave or later:

Update your iPhone, iPad or iPod touch with iTunes – Apple Support (UK)

See this link if you want to update from a Mac with Catalina or newer

Update your iPhone, iPad or iPod touch with the Finder – Apple Support (UK)

WiFi signal strength is way too low.

If you have a Mac or PC, try updating the iPhone that way.