Unable to update iPhone 7 Plus to iOS 15.8.4

I have read that iPhone 7 Plus will not update IOS 16 & above. I’m trying to update it to 15.8.4, I do believe it hat is below 16. Why won’t it update. It tells me I’m not on the internet. How am I typing this in Apple I I’m not on the internet? Is it me? Please help.

[Re-Titled by Moderator]