You might try mixing some biodegradable soap with water and pouring it into a spray bottle. Spray the insects and leaves that are ifected.

If the plant is stable and established, you can just wash off the plant using water hose. Aphids are very easy to get rid off. Aphids and other insects produce sugary substance that ants love and so protect these insects from their preditors. Once these insects go, Ants will be gone automatically.

I have Meyer Lemon (4ft) and was heavily infested with White Flies (which introduced ants, sooty mold,etc). I used Bugblaster (google it) device to spray water and it pretty much wiped them out. It's very effective. For white flies, though, you must wash them out every 5th day (not 4th, 3rd,etc.) to break their breeding/life cycle.

No need to use any chemicals, if you are persistent consistent w/ water spary. Good luck.

The yellowing leaves are likely to be caused by chlorosis, especially if the veins are green and the rest of the leaf yellowing. My Meyer lemon frequently must be treated with a chelated iron product. The last brand I bought was Black Label Iron and Chelate (I think that's the name). Get whatever chelated iron product your nursery carries.