The ants are farming scale. Watch for the scale - which look like little dots. Clean them off with rubbing alcohol and a rough rag. Then get some good worm castings to put around your lemon. For some reason it prevents scale.
It does not say what zone you are in. Lemons are hardy to 27 degrees.
I have a Meyer Lemon that is about 20 years old. It is in a huge pot. I bring it in to the greenhouse every November with a furniture dolly. The lemons ripen , then It blooms and sets fruit in the greenhouse.
If you live in a frost area, a few do not have a place to over winter the lemon, you can build a pvc pipe frame. Cover it with clear plastic and put some light bulbs in ther to turn on when it gets cold. The old kind of light bulbs that give off heat.