Plunger stuck inside tub drain

If the tub is blocked, then any CLR you put in the tub will be able to work on the plunger. but in general you will have to slowly drip the CLR over the stuck section, which will take a long time.

Something faster than CLR would be concentrated sulfuric acid drain cleaner. This is a unrefined acid, in a plastic bottle that feels like it is made of liquid lead, very heavy. I haven't seen it is a long time, but I used to see it in the stores in the 1980's. It was sold as drain cleaner, and was probably pretty hard on pipes, and people... so if you find it, be careful. Muratic acid may work as well.

Because it is in a drain, you loose the chemical as fast as you pour it in. One way around that would be to temporarly block the drain. Take a baloon, and some aquarium tubing, and a rubber band. Attach the baloon to the dnd of the tube, and inflate the baloon slightly to block the drain, and pour your acid of choice in. The acid may destroy the baloon, but at least you slowed the acid long enough to expost the plunger. Rinse and repeat as needed. Eventually you will either loosen the plunger, or decide it's time to open a wall behind the tub, or bust the tub up and remove the while thing, and start over.

Keep yourself protected from these acids. The sulfuric acid is extremely dangerous, and could come spewing out if it finds something fun to react with, so move slowly, carefully, and wear lots of protective gear. Keep a bucket of water with a large box of baking soda mixed in it for a rapid acid neutralizer ready before you open your acid bottle. Be ready for the worst, but stay safe for the best results. Work with a buddy, and keep the buddy out of the room but close by in case you need him to rescue you. Work out a rescue procedure in advance.