Peace be upon you and God's mercy and blessings.
Grand Ayatollah Sayyid Khamenei (may God protect him):
If someone intends to fast as a Qada (make-up fast) and deliberately remains in a state of major ritual impurity until dawn, their fast is invalid. If they forget to perform ghusl, their fast is still invalid according to the most cautious opinion.
As for experiencing a wet dream during the day, it does not invalidate the fast. If the fasting person knows that they will have a wet dream if they sleep, they may still sleep.
If a fasting person has a wet dream during their sleep in Ramadan or on any other day they are fasting, they are not required to rush to perform ghusl immediately upon waking.
Grand Ayatollah Sayyid Sistani (may God protect him):
The fast is not invalidated by being in a state of major ritual impurity at dawn, whether intentionally or not, whether it is the fast of Ramadan or any other fast, including Qada of Ramadan - even if the time is not tight - although it is advisable to exercise caution in this matter.
Similarly, the fast is not invalidated - whether obligatory or recommended, specific or otherwise - by having a wet dream during the day.
If one has a wet dream during the day in Ramadan, there is no obligation to rush to perform ghusl. They may also perform istibra (cleansing) by urination even if they know some semen remains in the urethra, but if they perform ghusl before istibra, it is preferable to delay it until after sunset unless they are certain that no semen will exit as a result.
May you be successful in all good.