The Danger of Error: 404 Page Not Found to SEO
Occasionally, it's necessary to have a website page on a different platform and server. WordPress may be better for adding a blog or portfolio to an existing website built on a content management system, although, both would be on the same domain. While this works well for most organizations, the portfolio or blog being down presents a special search engine optimization challenges.
High traffic blogs or portfolios shouldn't be down for more than 12 hours. People will try the page once and if they see that it is down, they will try back later. If the page that they want is still down, they may never return. The 404-error message makes it appear that the page is no longer available, giving human visitors no reason to try anymore. Moreover, search engines remove the page from their index when they encounter a 404 message.
Broken internal links are bad for the primary website's SEO and the visitor's user experience. Google wants to return the most useful results, and a site with broken internal links appears poorly maintained. If the blog has its own server and no one is sure what is wrong, remove the link in the navigation and remove all internal links as well, until the problem is rectified.
Dedicated hosting would solve the problem of maintaining servers in-house. While dedicated hosting can be expensive, you have someone monitoring the server and repairing it quickly if there is an issue. A few dedicated hosting companies offer automatic backups for WordPress sites.
While having two CMSs on one domain does not have any effect on SEO, it is often unnecessary. Enterprise level organizations are using WordPress to build business websites with a blog or portfolio. Updating the website and performing SEO is easier and faster, although the multiple CMS approach is common. For more information click here https://www.reddit.com/r/SEO/comments/8mvtvx/impactofablogbeingdownforwebsitesseo/.