if you create a new BACnet-Data-Source your local device IP (old system settings / BACnet local devices: 192.168.100.105:47808) should be the same as the Mango-Server has:
If you create a new BACnet data point it should look like:
where the "Device instance number" is the given Device ID, here 5.
If all BACnet-Devices are in the same IP-Range you don't need a BBMD.
Be carful if you scan a large BACnet-Network with many devices and objects. It can take a long time and need server-resources. Better is to scan one ID.
@hazel I am pretty confident one of your registers is being read as:
I have opened a ticket to make this more robust but the current version of Mango does not have a simple way to determine what data point is causing this. You will need to start looking into what registers you are reading for numeric values that have out of range data.