The next chapter is in the September issue not in September. It is a bit confusing, but the September issue releases on July 26th, there is a two-month difference. The previous chapter had mentioned August issue, but we are not in August yet.
Okay so the next chapter will release this month on the 26th. All the previous chapters were released on a monthly schedule. It says continued in the April month is because it will be released in an issue called the April Issue of Monthly Shonen Ace. If you check the last page of the previous...
If you have seen the anime read Ch 43(that story is a manga exclusive) and then start from ch 76. If you have more doubts about the reading order check the notes at the very end of Ch 51.