Jangan worry sangat lah, premature babies ambil more time untuk reach milestones, since your baby is borderline prem, dia probably still on adjusted age so technically baru around 5 bulan plus je! A few of my mum friends punya premature babies also tak meniarap sampai 7-8 bulan, so very normal. Keep doing tummy time consistently (maybe 5-10 mins a few times a day), tapi no pressure lah, rolling and crawling will come when baby is ready, ada babies skip tummy time altogether and proceed langsung to crawling. If you're really worried or notice other development concerns, boleh check with your paediatrician at the next visit, but honestly from what you're describing, sounds like your little one is progressing okay je!