Apple is saying my serial number is invalid despite I copied + pasted it straight from settings..

I wanted to check my iPhones service etc because I have an iPhone 7 and I can tell it’s on it’s way out due to it not being able to connect to Internet so easily, saying my storage is all used up even thought it isn’t and found out you needed to get your serial number to be able to do so, I found out how to do that and copied and pasted it into the ‘Check your service and Support coverage’ box however after numerous attempts it just keeps saying my serial number is invalid? And I’m really not sure why.