How do I restart an unavailable iPhone 6s Plus without a PC using the 2025 method?

As long as the 6s had been updated to iOS 15.2 or newer you can likely follow these steps:

(a 6s is capable of running up to 15.8.3)

If you see a iPhone, iPad, or Apple Vision Pro unavailable message or Security Lockout screen - Apple Support

Otherwise you’ll need to use a computer.

You will most assuredly need access to a computer. See what you need to do here --> If you see a iPhone, iPad, or Apple Vision Pro unavailable message or Security Lockout screen - Apple Support

If you don't have a computer, borrow a friends computer or visit your closest Apple Store Genius Bar.