Here are the websites they recommend. They are listed in order of difficulty, so I suggest you to begin with the first website and follow in a progressive order.
www.grammaropolis.com
www.grammarbook.com
https://elt.oup.com/student/practicegrammar/?cc=us&selLanguage=en
https://owl.english.purdue.edu/exercises/2/
www.grammarbook.com
https://elt.oup.com/student/practicegrammar/?cc=us&selLanguage=en
https://owl.english.purdue.edu/exercises/2/