Iphone Mode number doesn’t match the model on the box

You bought a gray market phone that was re-packaged in a different box.

If you're unhappy about it, return it and go buy a phone from an authorized retailer.

Rewrite:

Model A2105 is for most of the rest of the world except for North America. Not specifically for UK. It is vitally important to know where this gray market phone comes from, because that is the only place you can get it serviced under Apple's warranty if it needs to be fixed.

Have you checked it’s serial number to see how much warranty remains? You can do so at https://checkcoverage.apple.com/

The warranty starts the day of original sale at retail from Apple or an Apple Authorized reseller, so your iPhone of questionable origin may in fact not be “new” at all.

If you look at Apple's list posted by Michael Black model a2108 is a model made specifically for use in China. It has nothing to do with where some parts come from. All iPhones are assembled in China. There's no point in going to Macrumors when there is more accurate information available directly from Apple. To save you the trouble of clicking a link here is what APPLE says:

• Model A2108

• iPhone XR

• 1 (2100 MHz)
• 2 (1900 MHz)
• 3 (1800 MHz)
• 4 (AWS)
• 5 (850 MHz)
• 7 (2600 MHz)
• 8 (900 MHz)
• 12 (700 MHz)
• 13 (700c MHz)
• 14 (700 PS)
• 17 (700b MHz)
• 18 (800 MHz)
• 19 (800 MHz)
• 20 (800 DD)
• 25 (1900 MHz)
• 26 (800 MHz)
• 29 (700 de MHz)
• 30 (2300 MHz)
• 32 (1500 L-band)
• 34 (TD 2000)
• 38 (TD 2600)
• 39 (TD 1900)
• 40 (TD 2300)
• 41 (TD 2500)
• 66 (AWS-3)
• 71 (600 MHz)

China mainland

• China Mobile
• China Telecom
• China Unicom

Hong Kong

• 1O1O and CSL
• 3
• China Mobile Hong Kong
• SmarTone

Macau

• 3
• China Telecom (Macau)
• CTM
• SmarTone

I would return it and get a phone from an official Apple Authorized reseller. But it's your choice.

Unless this is an Apple Authorized seller, yes, you got scammed. There are few online only authorized sellers available in any country. You can find all Apple Authorized sellers with this search tool - https://locate.apple.com/amei/en/

BTW, there is no such thing, referenced by Apple, as an “international model” iPhone - iPhone - View countries with supported LTE networks - Apple