If you find your NAT Network is not working properly, the first thing you may want to do is list the networks, check their status and make sure the Network is actually started and configured as you expect (eg. is DHCP on and enabled?). This is a long known, unresolved bug that seems to affect Version 6 randomly and disportionately on especially Mint 20/Ubuntu 18. https://www.virtualbox.org/ticket/1474........