my phone number isn’t shown in my number

iPhone 5s, iOS 12

Try this: remove and reinsert the SIM card.

If it doesn't help visit your carrier, maybe the SIM card is damaged, ask for a replaced (new) card. Normally it is free.

Ask also to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.

Have you done the below?

Try this: remove and reinsert the SIM card.

If it doesn't help visit your carrier, maybe the SIM card is damaged, ask for a replaced (new) card. Normally it is free.

Ask also to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.

Is your phone number now shown in 'Mij number'? (your original question)

See -> If you can’t turn on or sign in to iMessage or FaceTime

If you can’t turn on or sign in to iMessage or FaceTime - Apple Support

Click on “My Number” and enter your number.

Do you have a phone number and a valid mobile account?

Have you tried restarting your phone?

If the SIM card is damaged then you cannot make calls and normal sms.

Can you make / receive calls and send / receive sms?