Statistics: Number of connected devices/day?

Hello !

I recently installed OpenWRT on my Fritzbox 4020, happily running Unifi APs in our guesthouse with various settings using both Controller and OpenWRT + Add-ons.

Now, the Wi-Fi provider we used to use (we still have a contract, but I already moved the APs to my router) is mainly using either OpenWRT or ddWRT custom (I forgot which) and they put some statistics, including the number of devices connected each day, on the customer account page. I attach the image. It is pretty useful in checking the trend/demand, so I would like to have that again.

I installed various statistics packages like collectd, luci-app-statistics, vnstat, but I couldn't find out if I can get such information out of them. Is it available from these packages, or is there something else which does that job ? I believe this Wi-Fi provider is using something within Open/ddWRT. (Or perhaps he made it by himself and not sharing it with the others)

I would appreciate your information !