Check settings - Access allowed by IP and Firewall
The Adminer needs port 80 and 443 to be open in order to be accessed.
Go to Virtual Machine -> Security -> Firewall and check port 22 and 443 are open for "Any Ip" or for your Ip.
If you wont to give access only to your Ip, then check your Ip here:
Allowed Ip access
Go to Application -> Settings, there is a section "Set Allowed Ip Addresses".
If no IP is provided, then any Ip can access the Adminer
If you set an IP (or a list) only those IPs can access Adminer
If your IP is not static (means it may change), then you have two options:
allow access from any Ip
set you Ip each time you need to access Adminer, for security reasons
After all those changes, accessa adminer URL