Issue Clicking a link breaks subdomain

[BrettFleeman]

Server operating system version

CentOS Linux 7.9.2009

Plesk version and microupdate number

18.0.63

Hi,

I currently have a main domain with multiple links pointed at a subdomain (hosted externally on a different IP)

The subdomain works perfectly either incognito or via direct URL, however when I try to access the subdomain via the links on the main domain, all that shows is the Apache testing 123 error, which then proceeds to break direct URL access as well.

Thankyou for any assistance provided

 

[BrettFleeman]

If any additional Information is needed, more than happy to provide
Subdomain is also hosted on a separate subscription so we can direct to the external IP