@Leanne13: I'm sorry to hear that you are experiencing this issue. If the problem has persisted following a Factory Data Reset, then there may be a potential hardware issue. Following the factory reset, did you install any third party apps onto your Galaxy A42? If so, please try removing them temporarily, just in case the fault is stemming from an external source. If this doesn't help, head to the Samsung Members app > Get Help > Diagnostics > SIM card, and check to see if this flags any issues. Although I understand that the SIM has worked in other devices, I also recommend reaching out to your network provide to request a SIM swap to see if this helps.
