No idea what im doing

The house I bought had 3 ubiquiti APs wired throughout the house. They have ethernet cables running into my modem. I thought I could easily buy a 4th one, had a wire run, plug it in and I'd be good to go. But I can't for the life of me find a way to discover the AP on my unifi network dashboard site. Should I punt on this idea and just add another modem?