old imac new iphone issue recognizing

Are you running iTunes 12.8? If you don't have 12.8, or want to make sure that you have the latest "point release" of it, you can find the kit here.

iTunes - Apple

Make sure that you're using a cable that's rated to carry data, not some "charging cable" that only carries power.

Did you get the advisory message shown in this Apple article?:

If your Mac says that a software update is required to connect to iPhone or iPad - Apple Support

If so, the "install " option ONLY updates iTunes for compatibility with newer versions iOS. It has NO affect on your current macOS version. Ive used that several times on multiple Macs without issue. The download is small and quick.

Hi,

Following article could help your issue.

If your computer doesn't recognize your iPhone or iPad - Apple Support