At this point you need to get back to the surgeon. You've likely scarred down heavily you will require manipulation under anesthetic. It happens to about 10-20% of the patient population. This is an unfortunate side effect to surgery that can happen, no fault of your own. If you are approaching and going beyond 10-12 weeks it's healed down and will likely not go anymore unless you are on the table, knocked out, and knee is bent to break down all those adhesions. I'm currently treating a guy right now who just had that done...and we are 2.5 weeks back at it and he's gone from a stuck rubber band 70 degrees to 110 stable. A full TKR should be done with physio by week 6-8, moving 110 deg by week 3 (or a full rotation on a bike with no hip hike). Complications are not that common for this, especially a young healthy individual, therefore this must be seen by your surgeon. The physio at this point is mainly modulating pain control, maintaining existing range, and improving whatever strength within the available range you have. But I have extreme doubts you will NOT be able to improve your range on your own, for said reasons.
I also want you guys to keep in mind your roles in this thread and others. I appreciate advice given but A LOT of this advice, whether it's here or in another thread, can seriously hurt someone by just telling someone, and may not be anywhere close to the advice they must be given, or something gone against what a physician has prescribed. We do not know histories, tolerances, co-contributors. This is pure physical therapy, anything medical must go back to their treating physician.