In newer versions of VirtualBox select from File menu > Host network manager. Create new adapter, and set the following options: Configure the adapter manually. ipv4 address 192.168.99.99 ipv4 network mask 255.255.255. DHCP server checked Server address 192.168.99.100 Server mask 255.255.255. Lower address bound 192.168.99.101 Upper address ...
In the Remote Desktop Preference window, fill in a name for the profile (like Ubuntu 16.04 Server), make sure that RDP is selected as the protocol (it should be selected by default), then fill in the host IP address - not the IP of the VirtualBox guest! - in the Server field, and select a higher Color depth than the default of 8bpp - 16bpp is ok.
For host-only networking, as with internal networking, you may find the DHCP server useful that is built into Oracle VM VirtualBox. This can be enabled to then manage the IP addresses in the host-only network since otherwise you would need to configure all IP addresses statically.
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Please note that the subnet 192.168.56.0 is a default configuration used by the VirtualBox as Host-only network connecting the host OS and virtual machines. The VirtualBox is also running DHCP server on this subnet reserving address range 100-254. So it is safe to use addresses below 100 for static addresses.
Probably, it is an issue with the default route. You don't need the Gateway and DNS for host-only network! You may check it using: ip route show. Remove them from the configuration, so default Gateway goes to interface with NAT/DHCP configuration. Also DNS which does not seems to affect, but keep reasonable clean configuration.