DD also attends a low achieving school and got AAAAABB in N5, but we had 3 tutors to help as there were teacher changes in several subjects, meaning she often didn't know what she was meant to be doing.
The study books are good, but they are best for exam revision as parts will be hard to understand without a teacher or tutor to explain them.
I would really recommend a tutor if you can get one. Rates do vary. You can have a searh on Facebook or post requesting one on a local page. You can also look on sites such as 'first tutors' or 'my tutor', there are some good tutors on there and their rates can vary from £20ph up to about £60ph. You can get university students, or actual teachers (I prefer actual teachers and ones who work in Scotland - make sure you read their profiles). You can expand the search within a 25 mile radius to get more matches and opt to recieve tuition online.
In addition to tutors, DD attended every supported study class that was available. She used YouTube - there's some good teachers on there who uploaded videos during lockdown so just search for 'national 5 maths', 'national 5 English' etc and see what comes up. DD found this really useful.
And there's bitesize website, scholar and e-scoil or whatever its called that is meant to be good.
Create a timetable for your DD with a subject each night on each subject (this is in addition to homework time). put in the timetable what she is to do. It helps to be organised and get into the pattern of studying. An hour a night should be ok. You will need to do your own research into what she is meant to know in order to create the timetable.
So for DD, I would do things like:
Monday - Maths - "watch youtube video one week on unit whatever", and next week "look at bitesize article".
Tuesday - "watch you tube video on Aunt Julia poem" one week, then the next week it could be "read bitesize article on RUAE".
Wednesday - Business Management - she had a tutor, so every Thursday would be "tutoring".
And so on.....
She had weekends off for time with her friends.