I say, and with God's success: Apostasy – may God protect us – is not one of the nullifiers of tayammum or ablution, so if a Muslim performs tayammum and then apostatizes, and then converts back to Islam, his prayer with that tayammum is valid; because what is achieved through tayammum is the state of purity, and disbelief does not contradict it: just like ablution, and apostasy nullifies the reward of the action, not the state of impurity. Refer to: Rad al-Muhtar 1: 170, and Sharh al-Wiqayah p1: 112, and God knows best.