Hi @fredrico You should have a Sim ejector Tool. Insert the pin in the tray hole to loosen the tray , pull out the tray gently and insert the sim with gold coloured contacts facing downwards and gently press the sim in the tray to secure it. See more details in Manual page 16-17 https://downloadcenter.samsung.com/content/UM/202105/20210513142713038/SM-A125F_UM_EU_RR_Eng_Rev.1.0... or see video
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My Device- S21 Ultra 5G ( SM-998B/DS ) CSC= BTU , 12GB/256GB
One Ui 5.0,Android 13 .