The secret to the crispy skin is to ensure the skin is dry. My favourite all time food video is this Marco Pierre’s slow roasted crispy pork: https://www.youtube.com/watch?v=DquBEjiQ0ys
I score the skin with a stanley knife using the chopping board as a straight side to guide cuts about 1 cm apart on the skin, be careful not to cut too deep. I put the belly in the fridge for 2/3 days uncovered skin side up to dry the skin. About 24 hours before cooking season the meat with your favourite seasoning, I often use 5 spice or just salt on the meat. I like to remove my meat from the fridge about 6 hours before I roast it. I cover the skin with veg oil and slow cook the belly for 2 hours at 160 no fan on conventional, bottom shelf, the meat is put on rack over a tray to catch the oil. Slow roasting makes the meat very soft and tender. The last 10/15 mins I turn the oven to fan with top heat, crank the heat up to 200 so the skin can bubble and crisp. You need to keep an eye on it as it will burn quickly.