I am not expert in planting but just some experience.

If you have been growing till such stage all by yourself then I should be learning from you. Over all the plants are thriving

When you say 2 months, I assumed you bought from nursery and it has been with you for 2 months. You know they are the experts and if they actually and professionally growing till this usable size then all of the plants should still be thriving and none should be dying. Some yellowing of leaves is expected.

You see what behind is that not all plant are suitable to grow in the same condition like this. But in order to attract customers to buy you have to have varieties.  Meaning have as many different species as possible, some big leaves some small leaves and some tiny leaves. But as said some are not meant to be planted this way. So, they take from pots and put it there and it should live but not thrive. Dont sound convincing? Ok you pluck a flower and put in a vase with water, that will last you few days to a week. You pull a plant with root and put it in a flower vase with water, it last you one month, 2 months? Ok this is only one of the theory.

When leaves turn yellow not due to aging it is because of too much fertiliser that "burn" the plant. Just like when you eat too much durian day after day you get "heaty" and fall sick. Too much of a good thing is not good. This will usually affect grass like plant with small long leaves. But why not the big leave? Size does matter! A 2 yr old kid eat one durian and a young 60kg young adult also eat one durian. It is not too much for the 60kg man. Ok this is also another theory only.

It is not over watering, over watering will have fugus growing. It is also not insufficient watering otherwise same shuould happened to all of them and the big leave should die first because it need more water than the smaller leaves as it has bigger surface areas.

The best way is to take this to the nursery where you bought them from. 2 things to do. First is to ask them why some are dying and, 2. Request they help you care for a month and pay them a small fee. After a month make sure they did not replace the dying plants with new ones. They will die (again). Sorry things dont die twice, you live once only

This plant requires good sunlight to thrive.
Generally, waxy leave plants dont require as much water, because the wax is to prevent water evaporation from the leaves.
If only one leave turns yellow, it could be either the stem is injured, or it's simply old. If bottom leaves r yellow, it could be they r prevented from getting sufficient lights by other leaves on top. If all the leaves are turning yellow at the same time, most likely they are not getting sufficient lights. If they turn yellow, stem getting soft, most likely there's injury to the roots, likely cause overwatering, not adequate drainage or fertilizer injuring the roots.

There seems to be a power cable.
It uses electricity ?

DIY?