He’s been in the prison world alone for over 10 years, more than likely dedicated — given he arrives in Mystic Falls in Legacies, he either got blood or this is a continuity error.
I would assume he found his way out of the chair and then proceeded to drink from blood bags that would exist in the prison world. Logically there is no reason he should have been stuck in the chair. Physically as a vampire he should have been powerful enough to break out. If they were spelled he would have simply siphoned them and broke out. So one way or another he should have been able to get out of the chains.
They were probably spelled as well as him being injected with vervain. Either that or the ropes were low enough that he couldn't move his hands to siphon the magic from the ropes