Since you're able to get to the homepage, I would expect the DNS and server mapping are setup correctly. In my experience, when someone can reach their homepage but not others, it's because permalinks are setup while the web.config (Windows) or .htaccess (Linux) file is either not present or not setup properly. These files can control many things in a hosting account, but with WordPress, they're most commonly used to control how permalinks work. Luckily, WordPress creates the files automatically.
Are you on a Windows server or a Linux server? Are you using permalinks? Does the problem correct itself if permalinks are disabled?
If you're on a Windows server, you should NOT have a site in a subfolder if you have another site using a web.config file in the root of the account. The web.config in the root will take control of the site in the subfolder which is not what you want. So, while I generally recommend Linux for WordPress (or any PHP-based site), if you *must* use Windows, be sure not to have one site in the root and another in a subfolder. Instead, all sites should be in subfolders of the root.
If you're using Linux, it's not normally a problem to have a site in root and others in subfolders. This is because of a difference in how a Linux server treats .htaccess files versus how a Windows server treats web.config files. If you're on Linux and disabling permalinks fixed the problem, you should be able to re-enable them (which will create the necessary .htaccess file) and have things work properly.
Let us know if this information was helpful. If not, I'm curious to know the actual domain name (if you're comfortable posting it publicly), so I could research your account directly.
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