Hello Knutsel77,
Welcome to the Apple Support Communities. I see you're looking for some specific cellular data reporting.
The location of your Messages cellular data use should be in the place you mentioned: Settings > Cellular > System Services > Messaging Services. If you don't see if in your list, you might double-check to see whether you're in the "Last Billing Period" tab or "This Billing Period." If you didn't use messaging services in either billing period, it may not show up there. Similarly, if cellular data usage was reset at any point, you may not see this show up: Check the cellular data usage on your iPhone and iPad.
I hope I've answered your question.
Cheers.