I could see that you're using the
/%category%/%postname%/ permalink structure.
Using your permalink structure, I have tested it in my local testing server. With Integrity stack and in a category page, the pagination works. (https://i.imgur.com/6YAiAwy.png)
There must be something that is preventing the default pagination to work in your site. This could be caused by a 3rd party plugin. You could try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
If the issue still exist, please re install X theme. There might be some files that were corrupted. You can review our update guide and see the
Manual Update section to re-install X:
Please let us know how it goes.