Hello and welcome, @luisnws. This sounds like a DNS record issue. I think this page can help: https://docs.github.com/en/pages/configuring-a-custom-domain-for-your-github-pages-site/managing-a-custom-domain-for-your-github-pages-site

GitHub Docs

Managing a custom domain for your GitHub Pages site - GitHub Docs

You can set up or update certain DNS records and your repository settings to point the default domain for your GitHub Pages site to a custom domain.

It could also be a missing file or configuration in the project. I see a GitHub 404 page when I visit luishenrique.space/lander right now.

If you need specific configuration recommendations, please let me know the actual domain and subdomain so I know what records to investigate.

I just reverted all DNS settings related to Vercel to restore access to my GitHub Pages. I would like to know how I can keep access to my GitHub Pages links (e.g., luishenrique.space/reponame) while setting up a subdomain (e.g., mysubdomain.luishenrique.space) for a project on Vercel.

I would not expect the addition of a CNAME record for mysubdomain.luishenrique.space to have any effect on existing records for your luishenrique.space domain. Just make sure you aren't changing or overriding the existing records.

You can follow these steps to add the subdomain to your project: https://vercel.com/docs/projects/domains/add-a-domain