What is the ruling of Islamic law and what is your advice regarding the divorce of a wife who embraced Islam then apostatized after 8 years of absence, and the separated mother wishes to relinquish my guardianship of my daughter who rejects Islam and avoids communicating with me?

It is not permissible to waive guardianship over the daughter, nor to leave her in the custody of her disbelieving mother, because a disbeliever does not have guardianship over a Muslim. Efforts must be made to reclaim the daughter from her disbelieving mother. If the daughter has not reached puberty, she is considered Muslim by virtue of her father. It is obligatory to strive for her guidance, keep her away from Christianization, and exhaust all available means for that, foremost among them being to remove her from the environment in which she lives.