What is the ruling on missed prayers for a person who mistakenly thought that ablution (wudu') could substitute for a full ritual bath (ghusl) after ritual impurity (janabah), and then realized his mistake after a long time?

It is not permissible for someone in a state of ritual impurity (junub) to pray with only ablution (wudu). Rather, a full ritual bath (ghusl) is obligatory for him to remove his state of janabah. If a person in a state of janabah prays out of ignorance, his prayer is invalid according to the majority of scholars, and he must repeat it. This is the ruling given in fatwas. He must also make up the missed prayers in a manner that he believes will absolve him of his responsibility, and according to his capacity, without harming his body or livelihood.