Can anyone help me to identify the problem, pest or disease. I have been treating it with Neem oil and bicarbonate but no improvement 

Treating when you do not know what the problem is is not a good idea. We need a lot more information, how have you been watering, where are you? Where has it been sited in your garden? I think it could have been caused by a lack of water at some time.

There are 2 variations of cotoneaster, one of which retains its leaves in winter, the other loses its leaves. It is important to know if the shrub was deciduous in the past. Then the discoloration may indicate that the shrub is going dormant and not some pest or disease.

Preparation for authum shedding does not look like this. This is clorosis. When the clorosis apear like this in older leafs only it is a symptom of a deficiency in mobile nutrients like magnesium, potassium, or nitrogen, as the plant moves these from older to younger, more demanding tissues.

The issue can be direct deficiency, that the nutrients is basically not there. But it can also be indirect, one nutrient in abundance blocking the uptake of another one. It typically happens if one have very hard tap water.

I would not do anything about it this time of the year. In spring I would do a half repot without damaging too many roots and water with rain water only. After the repotting stress start to give fertilizer.

Are we sure this is cotoneaster, leaves and grow habit looks more like azalea to me...

I am by far not a species identification specialist.

But if you are right it makes sense. Azaleas are strict soft water species and would react more and faster to hard water.