What goal have you been working towards and what progress have you seen in these 4 months?
If progress has actually stalled, I'd take it as a chance to ditch that app. I looked around and couldn't find any information about the qualifications of the designers, which leads me to believe it's, at best, a few guys who might lift and thought it'd be neat to make a workout app regardless of the quality of training info they put out.
If you've hit a plateau (and I'm not really sure how you're defining the term), more doing accessory work is almost definitely not the answer. But again, your goals are kinda important to know before determining the next step. Is the boxing for competition, for fat loss, or just as a hobby? How long are the boxing sessions? If they're like, under an hour of stop and go practice and learning, that's one thing. If they're 2 hours of non-stop hitting, that's something else.
What does your current nutrition look like? If you're having problems recovering, that would be something to look at before the training actually. Those 4 training days in a row, Monday to Thursday, could require more fuel to recover from, in terms of calories and/or macros like protein, carbs, or fat, depending on how you've been eating.