Should be easy enough to mount to the chimney. You can find antenna mounts with chimney straps, or if you want a more permanent install, tapcon a mount to some of the rocks. That will require a hammer drill tho. An example of chimney straps but you may need longer ones given that's an absolute unit of a chimney. Also make sure to think about lightning protection having it sticking up that high. https://www.amazon.com/Chimney-Mount-Galvanized-Steel-Straps/dp/B010OMXZ14/ref=sr_1_3?keywords=chimney+antenna+mounting+kit&qid=1662846444&sr=8-3#customerReviews

​

​

And that pipe is absolutely not unused, that looks like a vent stack pipe. Do not block that off or enjoy your toilets/drains not draining right. Plus it won't be strong enough to support a station anyway.

Amazon:
https://www.amazon.com/gp/product/B010OMXZ14/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Pretty sure you can get them at homedepot/lowes, you might just have to order online.