I had the same thing happen to mine. It is not glued in I can just about promise you. Tanks are generally polyethylene and sewer fitting are ABS, so gluing them in is not an option. Mine had a one piece flange with 3" NPT that screwed into the tank. It was tight as heck largely due to the fact that some type of non hardening caulk was applied to the threads. I had to take a heat gun and heat up the threads from the inside(oh yeah, smelled great!), then I could unscrew it.
https://www.shelllumber.com/united-states-hardware-p-110c-plast-m-toilet-flange.html?feed=Froogle&gclid=Cj0KCQiA-K2MBhC-ARIsAMtLKRvb0khxziBLSkzUn00TPdprJTxcOs8GqIDKvxkLvoivw5RnqpES_WUaArC9EALw_wcB