The SIM card is not registering on the phone

This should fix the problem: https://support.apple.com/en-us/HT201420

Failing that, take it to your nearest Tesco store (not Tesco Express). You may have to queue, so make it worth the wait by getting a few essentials in.

It's quite possible that the phone was shipped without a sim., but you can probably use the sim from your previous phone if you still have it.

You can use any sim card from any supplier, but not all sim cards are the same size. not all phone shops will be open.

Is there a sim card actually in the phone? If so, all you need to do, if you don't have the little tool for opening the sim card drawer, is to use one end of a paper clip (the metal ones).

Make sure your phone is securely switched off for this procedure.

If you open the pin out, you only need to open the outer end of it and push that into the hole in the sim card drawer.

The hole is about halfway down on the right side of your phone.

If the sim card is in there, it may not be inserted properly. It needs to be inserted with the contact side face DOWNWARDS.

DO NOT touch the contacts as any residual grease from your fingers can stop them from working. When you are satisfied that the card is properly seated in the drawer, close the drawer and turn your phone on.

One last word; your phone has dual sim capability:

Dual SIM (nano-SIM and eSIM)13

Not compatible with existing micro-SIM cards

1. Use of eSIM requires a wireless service plan (which may include restrictions on switching service providers and roaming, even after contract expires). Not all providers support eSIM. Use of eSIM in iPhone may be disabled when purchased from some providers. See your provider for details. To learn more, visit support.apple.com/en-gb/HT209044.

This could be the reason why you got the peculiar response when you turned the phone on.

Then you will need to purchase new. Remember, the 11 Pro has dual sim capability.

I've looked on Ebay and Amazon. Neither offers sim cards for sale. You may need to go to an iPhone seller.( doesn't have to be Tesco) It's all down to whether you want to go on a Pay As You Go or a contract.

Remember this?:

Dual SIM (nano-SIM and eSIM)13

Not compatible with existing micro-SIM cards

1. Use of eSIM requires a wireless service plan (which may include restrictions on switching service providers and roaming, even after contract expires). Not all providers support eSIM. Use of eSIM in iPhone may be disabled when purchased from some providers. See your provider for details. To learn more, visit support.apple.com/en-gb/HT209044.

If you don't want to go down that route, then a standard micro sim should suit it, but ask about what's on offer in terms of data , texts and calls, and if you can, go to a Three, Vodaphone or EE shop to get the best deal you can.

Best of luck. I'm still here watching responses.

Apple don't make an iPhone 12....yet. You quoted your phone as an iPhone 11 Pro.