Preheat your toaster oven to 350°F (175°C) before you begin. While waiting, lay the wings out on a piece of aluminum foil or a toaster oven tray. If your toaster oven has a grill function, use that setting to mimic the high heat of an outdoor grill.
Then arrange the chicken wings in a single layer, giving each wing enough space so that they're not touching; this ensures even reheating. If your toaster oven has convection settings, use them; the fan will help circulate hot air around the wings, making them crispier.
Heat the wings for about 10-12 minutes, depending on their size. Halfway through, flip the wings over to make sure both sides get to crisp up.
In the last couple of minutes, you might want to switch the toaster oven to broil to give the skin that extra crispiness. Just keep a close eye on them so they don't burn, especially if coated in a sweet sauce. Once done, use tongs to remove the wings from the toaster oven, as they'll be hot.