Is the ijāzah (certification) I received from a shaykh who granted it to me after a complete recitation of the Qur’an online by reading from the Mushaf (physical copy of the Qur’an) valid, and can I use it to grant ijāzah to students based on it?

It is not valid for someone who recited to his teacher while looking at the Mushaf (copy of the Quran) and was granted an ijaza (authorization) based on that, to then grant others an ijaza that is considered valid in the terminology of the reciters. This is because a legitimate ijaza requires reciting the entire Quran from memory to the authorizing teacher. Nevertheless, there is no impediment to giving a certificate to someone who has mastered recitation by looking at the Mushaf, even if it is not an ijaza in the known terminology. If you desire a valid ijaza, you must recite to your teacher from your memory. However, not obtaining an ijaza does not prevent one from teaching the Quran if one is qualified to do so.