The Local Area Network, or LAN, can and will achieve 100MBps with cards as old as ~1990 give or take ten years depending on what card and network it is.
The Wide Area Network, or WAN, is what the ISP controls.
To visualize the idea, think of the WAN as the jug of milk.
Now think of the opening of the jug of milk as the Network card in your machine in this instance.
Lots of milk in that jug, the size of the opening, where the 'cap' / bandwidth is, will determine at which the flow of milk runs from inside the jug, to the glass / LAN.
Just as; lots of available 'bandwidth' or available flow on the WAN, where settings or size of the 'cap' or capacity of flow in the network card to the local machine.
Where are you reading the 100MBps from? If it is hovering over an icon in the lower task bar in windows, that is the current maximum theoretical speed the Local network, or LAN can achieve from the hardware / software which controls it.
Where are you reading 'Not enough bandwidth' ?
And what are you attempting to do when seeing this message?