One tip, after toilet training several dogs, is to pick up the mess from the "wrong" place and put in the "right" place, let the pup see, then bring her to the wrong place and show her with a firm no then over to right place and a "good girl" etc....... It Can take between 2 to 6 weeks of hard work but well worth it !
I have only known a few boxers but they are a great dog, 2 "problems" can be "wind".....can be an issue with all big dogs but boxers can be especially bad ! the other is a health issue - they can, when older, develop some heart problems, but in the cases I know this can usually be controlled with tablets.
Best of Luck -Enjoy!