How do I fix /page/ indexing?

The easiest solution would be to install an SEO plugin. One of the best IMHO is the SEO Framework plugin which offers a relatively light and I believe ad-free solution. See the screenshot of how easy it is to set pages to no-index.

Good luck.

It will be better if I get the robots.txt solutions

If you feel more comfortable in working with the robots.txt file directly rather than through the SEO framework or a similar plugin, that’s great.

Good luck.