keyless remote battery 07 nissan altima

Recently a light came on in my Nissan Altima 2007 indicating what I thought meant that I needed to replace the battery in my keyless remote. So I ordered the battery online and went to a dealer for them to install it. But now the light is still on- it went away temporarily after driving a while- but then came back on? How can I get rid of this light indicating that my key battery is low? Or have I not solved the problem? The dealer mentioned that I would need to replace both key batteries, but I bought the car used and it only came with one keyless entry remote. ???