I get where you’re coming from. Creating a fake account is the first thing most people try, but getting blocked shuts that down quick.
I haven’t tried the monitoring software that others have mentioned, but I did have some success with the airplane mode trick. Basically, you open Instagram, let the story load, then switch on airplane mode before watching it. Just make sure you close the app completely before turning airplane mode off again. It’s not 100% reliable, but it’s worth a shot.
Another thing I’ve done is ask a friend who doesn’t know my ex to check their stories for me. They can take screenshots and send them over. It’s a bit roundabout, but it works!