How many devices are connected ?
How many computers ? How many IoT devices ?
Your bandwidth is shared by all of the connected devices. Any device connected will use some bandwidth and this can easily add up very quickly, especially for a «smart» home setting where you can have every wall switch, wall plug, light bulb, TV, baby monitor, smart lock, door cam, fridge, oven, washer, drier and many other things all wanting to communicate in some way.
Nobody really need a connected fridge, oven, washer or drier anyway, and they should not demand any kind of connectivity.
Unless you can't do without, you should not need the uPNP feature. Turn it OFF on your router.
Change the password for your Wi-Fi. Maybe someone in the neighbourhood is using your connection without your consent. Make it at least 12 characters long. Use one or more non-alphabetic character.