I planted a lemon tree but its not increasing from last 2 month. Don't know the issue

Give it time for the roots to establish them self's, don't let the soil go dry.

I'm concerned about the yellowing leaves; it appears to be some kind of possible disease. Is there a gardener nearby who grows lemons that could help you out?

As they said, you have to give it time. And while there can be other reasons for lemon leaves to turn yellow, one cause can be simply a sudden change in nutrient profile, such as planting out. I suspect that one, because it's rather sudden for a newly introduced plant to acquire disease. And your tree was almost certainly grafted onto deciduous stock, so it will go through seasonal changes in fall. It takes up less nutrient and can show it. And that might be compounded by the first suggestion.

I wouldn't worry about lack of growth in only two months in winter - roots always grow first. But I'm with Gardenfish, it looks like a disease or fungal infection.

It appears to be a seed grown tree.

Thank you so much everyone for your valuable suggestions.
I will wait for some time