Apple TV 2nd generation
is there a way to use my iPhone as a remote for a second generation Apple TV?
Not unless you still had the previously available remote app. Even then, if you lost the physical remote then you would be unable to pair.
FYI: The digital remote is now in control centre and requires a 3rd gen or later. This model has long been obsolete, most providers have dropped support and given its age it's seeing more issues in operation.