I helped a family member to get access back to his instagram account which also got hacked and got his username changed. What we did was follow the instructions

I helped a family member to get access back to his instagram account which also got hacked and got his username changed. What we did was follow the instructions in https://help.instagram.com/contact/740949042640030 https://help.instagram.com/contact/740949042640030 and kept in touch with the instagram staff via email. They kept asking various security questions during email and finally sent a recovery link.