and you dont go by what your router says, router is overall network activity which will be hugeeeeee, to check true in and out data you would login in to your internet companies website like you are going to go pay your bill then go to internet usage which will probably be around 500gb to 600gb per month or less if what you are saying is 100% accurate

perfectly normal might seem a bit high but this day and age with like a fiber connection everything is connected to the WWW and of course with a fiber connection thats nothing its just like breathing air

like i did it myself by using extra router and 75 feet of ethernet cable ran at the top of the wall, so a router to router connect with easy ran wiring saves a ton of money