Strength training with a weak back

Hi all I want to start strength training but I have had a weak back since a bike accident I had years ago. I also have Fbm syndrome. I can't afford a personal trainer but I was looking into getting some resistance bands to use. I want strengthen my core as I know it will help my back but situps or crunches are a no go for me. Does anyone have any ideas for what I can do and what resistance lvl I should start at?