It looks like your tree has several somewhat serious problems.

Nitrogen deficiency causes lower leaves to turn yellow first. The tree is probably low on nitrogen. A liquid houseplant fertilizer is better than the spikes, since fertilizer dissolves very slowly from those, and it does not move laterally.

The leaves are also showing interveinal chlorosis, probably due to a deficiency of iron or manganese, which would be unavailable to the plant if the soil pH is too high. Added peat moss in the soil mix could help, along with spraying the foliage with a chelated iron spray. Do not add lime to the soil mix.

You may also have some insect problems, such as leafminers, aphids or scale (a common problem on citrus). A systemic insecticide soil drench registered for use on houseplants would be an option, but if you can't find that, an insecticide soap may help control some insects. Check label to be sure the product you buy can be safely used on citrus. If the plant continues going downhill, you may be better just starting over by purchasing a healthy plant, but first, get the unhealthy plant out of the room where the new plant will be located.