What is the ruling on performing the obligatory prayer with ablution (wudu') instead of tayammum or ghusl, when water and a mosque are available, but ghusl from janabah is not possible at that time?

As long as you can find a mosque and water, you are able to perform ghusl (ritual bath), and ghusl is obligatory for you. It is not permissible for you to perform tayammum (dry ablution) or suffice with wudu (minor ablution). If you pray without ghusl, you must repeat the prayer.

For one who is unable to perform ghusl, tayammum is to be performed. It is not permissible for him to perform wudu instead of tayammum according to the majority of scholars, because wudu does not remove janabah (state of ritual impurity requiring ghusl). The opinion of those who permit wudu instead of tayammum is considered an outlier.