Highlight text in reading mode

I'm an avid reader of books, nd recently I have developed this habit of reading in mobile as it helps me to read even when I'm out, where obviously I can't carry books. Nd I like to highlight the important lines or paragraphs so that it helps me to revise easily when I'm reading second times, but I'm not able to highlight text in the read mode and even if I have highlighted in the page mode when I change to read mode to ease my reading experience the highlight ink vanishes and it will be as if I'm reading all new lines. Please let me if any troubleshoot is there for this issue, if not please make it available. Hoping to resolve as early as possible, thanking in advance.