Will iPhone XR work on 5G

It won't use 5G specifically (that requires a different cellular modem), but there is no way that 4G LTE is going away any time soon.

No.

It doesn't have a 5 G modem in it.

The iPhone XR will continue to function when 5G is introduced. There is no iPhone that has been made that will take advantage of the new 5G capabilities.