Try soaking in PlusGas, for a good few hours at least, and I mean drench it.
Then, get a stilson, and a length of pipe which will fit over the stillson handle. Thick wall tubing like steam pipe or scaffold tube is best. Now secure the frame as tight as you can.
Slip pipe over stillson handle and apply even ,measured pressure until the teeth of the jaws startvto bite. If you’ve never used a stilson before, its critical you get the orientation of the jaws correct. Checking they bite is crucial, if they dont feel solid, rotate the wrench 180 degrees ie flip it over. Reapply pressure, evenly and gradually. Do not snatch at the stilson.
Leverage is king here, work smarter not harder. If its still no go, increase the length of lever you are using on the handle. Onceyou get movement, drench in PlusGas again and repeat. Try to use as short a lever as you can get away with to prevent the chance of the post shearing off in the frame.
I fix press tooling for a living, 10 tons plus, and we can move these with a 5′ bar using leverage.
Good luck.