Dog is territorial on walks but we live in an apartment!

I adopted my boy Charlie about 2 years ago. He's somewhere between 3-5 years old. 20lbs dachshund mix.

Back in January I moved from a house to an apartment. When taking Charlie on walks, he was initially fine but after about a month he started barking excessively if he sees (or sometimes even smells) another dog, but ONLY when we're walking around the complex.

He gets along fine with my other dog. If we go somewhere else, he doesn't bark at other dogs. He also goes to doggy day care 2 days a week and the staff always tells me what a delight he is! I can also see him on the camera and he's very clearly not aggressive or barking at the other dogs.

Does anyone have any suggestions on how to get him to chill out on our normal walks?