Yellow leaves are normally either a lack of water or a lack of nutrients. It could also have too much nitrogen. Make sure you are watering correctly and give the tree some slow release fertilizer. Additional phosphorus can hellp with fruiting. These articles will help with fertilizing your tree: https://www.gardeningknowhow.com/edible/fruits/lemons/learn-about-fertilizer-for-a-lemon-tree.htm
https://www.gardeningknowhow.com/edible/fruits/citrus/fertilizing-citrus-trees-best-practices-for-citrus-fertilizing.htm