I loved this show and it's sad that it is over already so I'm wonder at which chapter I can pick up at.
I've searched for it and it seems they adapted chapters/gags from anywhere in the manga, so it's a bit of a mess. The summer festival from episode 12 comes from chapter 80 of the manga, but the 'Nasa having a cold' part comes from chapter 88. What's worse is that they skipped everything from chapter 44 to 80, so you're missing out a lot of content.
My advice is to start reading from the beginning, it won't consume much of your time since chapters are somewhat short.
Haven't read it but they probably picked important chapters and the other plot lines that they thought would do the best, from personal experience It's likely you will feel like you're rewatching some of the show but there are probably 10s of chapters to miss from skipping to the anime ending point.
I've searched for it and it seems they adapted chapters/gags from anywhere in the manga, so it's a bit of a mess. The summer festival from episode 12 comes from chapter 80 of the manga, but the 'Nasa having a cold' part comes from chapter 88. What's worse is that they skipped everything from chapter 44 to 80, so you're missing out a lot of content.
My advice is to start reading from the beginning, it won't consume much of your time since chapters are somewhat short.
I'd usually say use https://www.mangaupdates.com/ and it should tell you, but its not up to date.