Old lower leaves on a rubber plant turn yellow or orange when the plant is adjusting or dropping older growth. The rest of the plant looks healthy and firm so this is usually normal aging rather than a disease.

Your watering schedule is very long for a young plant. The soil may be drying out too much between waterings. Water when the top inch feels dry, not on a monthly clock. Better consistency keeps new leaves strong.

You can remove the yellow leaf. It will fall off soon anyway.