Model or serial numbers for fridges and freezers always used to be on the back (as with many other appliances). My own fridge only has its serial number on the back. Fortunately most manufacturers have now realised they should be much more accessible.
On a freezer
A model and serial number on a modern freezer should be on a label inside, usually stuck to the side somewhere. Open the door and have a good look around. It’s sometimes at the base but it could be anywhere. Remove drawers from the bottom of the freezer one at a time until you find it.
On a fridge
The model and serial number on a fridge is often on the inside wall side next to the salad drawer. Remove the salad drawer to find it.
On a fridge-freezer
On a fridge-freezer the serial number may be in the freezer compartment.
If you can’t find serial number
The rating plates are usually silver but can be white, and are usually on the left side. If you can’t find these numbers and you’ve looked at every conceivable hiding place inside you may have to pull the appliance out and hope there’s one on the back. The only other place one could be is if there is a plastic kick strip at the base of the appliance that may be easily removed and which could conceal a label.
If pulling the appliance out be careful. Few things damage flooring better than heavy fridges and freezers with no wheels fitted. One tip is to push it onto its back feet and place a towel or something under the front feet to help slide it out.
Take advantage of moving it out to give the appliance a good clean out at the back especially inside the back of the appliance and round any grills. Blocked air vents in fridges and freezers caused by a build up of fluff and debris cause 44% of fires according to one Fire brigade.
Finally: model and serial numbers must be on an appliance somewhere. It’s highly unusual for one to be missing. There’s no logical reason why a manufacturer should deliberately hide it from us. If you cannot find the numbers then contact the manufacturer and ask them where it is.