The first thing is to accurately diagnose what your lemon tree has. Spider mites? Citrus or some other type of citrus-specific mite? Or perhaps something else entirely. Can you send close-up pictures of the critters? If they are too small to view effectively, look at them through a lens and describe them as well as you can - color, number of legs, how mobile they are. Also any symptoms your lemon is showing.

Hi,

Thanks for the excellent photos. What your lemon tree has is scale, which is something citrus are fairly prone to. Scale insects grow a hard shell to protect them from predators, and it also provides pretty good protection against insecticides. When they first hatch out and are mobile, they are called "crawlers" and are more vulnerable then.

This also explains the ants. Like aphids and some other sucking insects, scale insects excrete sweet "honeydew", which ants like to eat. They will hang around and protect the insects from predators.

It looks like your tree may be fairly small, and the infestation not too well established. In this case, your best bet is to scrape them off with your fingernail or a piece of plastic - something that won't damage the plant. Then keep a sharp eye out for re-infestation.
Controlling the ants will help by allowing predators to get at them - this is most helpful earlier in the spring when they are crawlers. You can spread petroleum jelly around the base of the trunk to prevent ants from crawling up. 
https://pnwhandbooks.org/insect/hort/nursery/common/nursery-crop-scale