'Tis an old post resurrected, but JMD-tech may have a slight disconnect over where the problem lies. Though there is not enough information in JMD's post for a complete diagnosis.
Bridged is the optimum setup for easy access between the VM and the LAN. But
things outside of Virtualbox's control can prevent Bridged from working.
See
Virtualbox Networks: In Pictures: Bridged Adapter.
JMD-tech wrote:VMs for development, on a laptop host that frequently changes from a network to another,
I gather from usage of a laptop and different networks that Wi-Fi is being used.
If Bridged is connecting to a Wi-Fi adapter, then the Wi-Fi adapter drivers and the Wi-Fi access point firmware could interfere with Bridged. Technically,
the Wi-Fi protocol does not allow Bridging. Some Wi-Fi adapter drivers and WAP firmware are lazy and don't implement the complete protocol. These situations allow Bridged. Strict drivers & firmware block Bridged. Virtualbox has absolutely
no influence on what drivers or firmware are being used. Thus Bridged works in one host or place but not another.
Bridged always works on a properly-working wired Ethernet adapter.
Regarding the OP question: NAT and NAT Network act just like house routers. These network types, just like house routers, have port forwarding, which allow access to the VM from the host or internet. See these types in the linked "In Pictures" tutorial.
JMD-tech or others with further questions, please start a new topic for further discussion.