I have a whole list of things wrong with my lower back. Biking is one of the things that really helps it. I'd be curious why your PT forbids it, as my experience and that of most folks is exactly the opposite. When you bike, your position should feature a torso inclined at 45° to 50° to the horizontal and a straight back. No or little curvature to the lower back. Right flat from butt to shoulders. It is very bad to bike sitting bolt upright on the saddle.