Air Carrier First Officer reported failure to notice glideslope out of service resulted in ATC low altitude alert; low PAPI indications.

Narrative

Coming in for a visual for Runway 9R in Atlanta and using ILS as a backup; I was descending in vertical speed because we had been a little high. I set the FAF altitude; but we hadn't captured glide yet; and I didn't want to level at that altitude so I set the missed approach altitude (above us) and continued down. As we descended I realized we weren't getting a glide slope reading and saw that we had four red on the PAPI. The Captain mentioned he thought the glide slope was broken and I turned the Autopilot off to hand fly and level out. Tower gave us an altitude alert and advised us to maintain 2100 and we leveled. I stayed level until the PAPI showed 2 red and 2 white and continued down for a safe approach and landing. Cause: Still being new on the aircraft; I didn't catch the glideslope not registering right away. Starting the approach slightly high. Suggestions: I should have noticed the glide slope wasn't registering. I had been using the green ball descending on the arrival and the beginning of the approach but I failed to notice the glide was never there. I should have also paid closer attention to the NOTAMs because it was noted out of service.