If the user declines the ATT prompt, can we send them to Settings to change it later?

Yes, it is possible to link the user into the Settings page to change this the ATT setting, if the user hasn't disabled tracking requests at the device level. Apple even mentions this here: https://developer.apple.com/app-store/user-privacy-and-data-use/.

It won't reset the prompt to show a second time, but the user can go there and manually change the toggle setting.