Who I have the same questioned this topic

Just received the Galaxy S8 to replace my S5. When I am on the phone, I am unable to receive mobile data.  i.e.; Apps won't work while I am on a call.  I get a pop up display that says I won't be able to receive mobile data while I am on the phone.  The cell bars gray out and all of my apps stop working.  I turned on advanced calling in settings, but it is still not working.  My S5 had no problem doing this.