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It sounds like you may have an issue with mites or scale bugs. The use of neem oil is very effective for treating these pests and is perfectly safe for plants, people, and even beneficial insects. Here is an article with more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm Once the tree is no longer stressed from the pests, it should begin to perk back up and produce again. You may also want to give it some fertilizer. This article will help: https://www.gardeningknowhow.com/edible/fruits/lemons/learn-about-fertilizer-for-a-lemon-tree.htm

You do not need another lemon tree to produce fruit. On average, these trees begin bearing fruit around four years of age.

I would try giving it some phosphorus rich fertilizer, like bone meal. If it is lacking phosphorus, it will not be able to produce blossoms (which means no fruit.). Here is more info that should help: https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-not-fruiting.htm

I would try giving it some phosphorus rich fertilizer, like bone meal. If it is lacking phosphorus, it will not be able to produce blossoms (which means no fruit.). The requirements for lemons are very similar to that of limes. If you have not read it yet, this article may also be helpful: https://www.gardeningknowhow.com/edible/fruits/lime/reasons-and-fixes-for-lime-tree-not-producing-blossoms-or-fruit.htm

Your tree is most likely shocked from the move from pot to ground.
Yellowing leaves are generally a watering issue, either to much or to little.

A newly planted tree will need daily water for the first 2 weeks and with temperatures above 85 degrees you will most likely need 2 waterings a day.
Make sure the tree was properly planted and that the soil is adequate for the tree success.

Add 2 to 3 inches of hardwood mulch around the base of the tree to help it retain moisture.

Here is a link to refresh you on the care and growing requirements.

https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

Yes, and this is likely due to a phosphorus deficiency. I would fertilize each spring with a citrus fertilizer, dolomitic lime, and iron sulfate. This will give you tree everything it needs to bear fruit correctly.