Put some ice and water in a plastic bag and sit it on top of the cap. Let it sit there long enough to cool down the cap.

While the bag is sitting on it, pour hot water on the ground against the pipe to heat up the pipe.

The cool cap will contract while the warm pipe expands. This might loosen it enough to use a straight bar to unscrew the cap.

Edit:

I'm guessing that the real tool would be something like this:

https://www.grainger.com/product/SUPERIOR-TOOL-Drain-Wrench-Rubber-45W758