An incorrect default gateway on a device typically leads to the inability to communicate with devices outside the local subnet. The default gateway is responsible for routing the packets of a device to destinations not within the immediate network. If it is incorrect, the device can only communicate locally, not beyond its subnet, which is why the device 'Cannot access external websites or services' is the correct answer. The other options could be symptoms of other configuration errors, but they do not directly relate to problems with the default gateway setting.