Once you create a valid FQDN for the server and DNS resolves it, you will have to edit config.php AND also do a admin search and replace on the DB via http://FQDN/admin/tool/replace/
Search for the error 'shortname' with http:// infront and /moodle/ at the end ... replace with the FQDN url including the /moodle/.
Assignments are one of the modules in Moodle that does a more strict check on DNS and valid URL's.
If the server doesn't need to be accessed from outside the internal private network and you are the only person evaluating, one could use the 'host' file of a workstation to associate IP address with a FQDN that's not in DNS for everyone to discover/use. I do that often in evaluating newer versions of Moodle ... same server ... just different config for a virtual apache by another hostname ... dev.fqdn.
IF interested: https://www.howtogeek.com/howto/27350/beginner-geek-how-to-edit-your-hosts-file/
'spirit of sharing', Ken