Yes but it would have to be done at the router level.
With a properly setup decent quality
router this configuration is possible. You probably cannot do it with the routers you get free from ISPs - would need a Draytek or similar.
Also is it that simple - with a monthly usage cap how would you share it between weekly (or shorter) lets? The first let of the month could use all the allowed usage.
Could charge separately for internet access to cover the cost?
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