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.
If the below doesn't help or work the best course of action is to power off the VMs, create a new Nat Network and assign the new Nat Network to the VMs and that normally fixes it.
List NAT Networks:
VBoxManage natnetwork list
1 network found
Start NAT Network:
VBoxManage natnetwork start --netname
Stop NAT Network:
VBoxManage natnetwork stop --netname
Create NAT Network:
VBoxManage natnetwork add --netname
Delete NAT Network
VBoxManage natnetwork remove --netname
Make changes to NAT Network
VBoxManage natnetwork modify --netname
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