The Deutsche Welle App is an excellent place to start if you’re looking for a German grammar review, or to improve your German skills on the go.
Start with an “Einstufungstest” (placement test) to figure out which level you’re in. Deutsche Welle features A1, A2, and B1 exercises, that is, from beginner to intermediate. Each placement test features 30 questions. 
The “Übungen” (exercises) in the placement test can also be found in the “Alle Kurse” (all courses) section. These are divided into levels, beginner through intermediate, and contain exercises focusing on grammatical concepts.
The Deutsche Welle App also features an overview of grammar, including verbs, adjective endings, and cases. These short summaries are similar to what you’d find in a German 100 or 200-level textbook, and are divided into bite-sized pieces.
The grammar review section (Grammatikübersicht) gives clear, short descriptions of the grammatical concepts. Although they aren’t as thorough as college textbooks, they are useful for reviewing.
Have fun learning!