21 pound brined turkey has been in a 350 oven for two hours, internal temp is 154 & my digital thermometer gives pre alert

Hello Helen! For a bird of this size you may want to consult this handy guide https://food52.com/blog/18372-the-essential-numbers-a-guide-to-times-and-temps-for-cooking-your-bird. Since it usually cooks for at least an hour after you’ve taken it out of the oven, plan to bring it out close to the desired temp so it won’t be overdone. Hope that helps!