every time I create a sitemap, the status that appears starts with Gateway timeout 504. when I refresh, the status appears as per the documentation but the numbers there don't change like in the demo. and every few minutes I have to tick to continue the process.
and then sometimes the process is inconsistent, like being in depth 3, after so many times I refresh again to depth 2.

it looks like your server configuration doesn't allow to run the script long enough to create full sitemap. Please try to increase memory_limit and max_execution_time settings in php configuration at your host (php.ini file) or contact hosting support regarding this.
memory_limit and max_execution_time have been increased. but I decided to delete this plugin first. after I deleted it, it still got error 504, then I deleted sitemap_news.xml, sitemap_image.xml, etc and user IIS_IUSRS. after that no more 504 error.
does this plugin run services while crawling especially when run in background is active?