+1 to what Emma said ...
DNS, however, might need an entry in two DNS servers ... 1 for the internal private network and resolves the 192.168.20.x IP address to a fully qualified domain name ... like: elearning.somenetwork.net - and in an external DNS server that resolves public IP addresses ... your 201.140.105.5.
Those are not necessarily on the same DNS server.
Change the config.php file's $CFG->wwwroot value to point to the FQDN of the server. If you have developed courses/added links to uploaded files, then the search and replace tool needs to be run AFTER you get the FQDN setup and working.
DNS setup doesn't take long IF your network people are the DNS server admins. Internal DNS happens quickly but external might take a day or two to propagate.
On a temporary basis, if you know what the FQDN will be, one could make an entry in internal machines host file (on a Mac found in /etc/hosts) that maps the 192.168.x.x IP address to the FQDN of the server. External access could be gained the same way, but best to setup DNS properly.
'spirit of sharing', Ken