OK. Sorry for not explaining properly. I do it for people as a side hustle. I first bridge your current router and I'll install a new router that runs a version of openwrt. It allows you to set data quota either per device or per user (this is good for users with multiple devices). It also allows you to set what happens once a user depletes their daily allocation, either shutdown internet or throttle them. And also, it can also set guest network so you can give your guests allocations for each of their devices. That way they don't blow through your monthly cap in a few hours.