HG659 limitations on connected devices

OK, I'll try to explain as best I can, what the problem is (I'm a dinosaur at this stuff, so please go easy)

At work (a Digital Photo Lab) we have Vodafone Fibre and a Huawai HG659 Wifi Modem/Router. We use an implementation of Dropbox to allow customers to connect their smartphones to our Wifi to upload photos for printing. On busy days, we can end up with 80 to 100 people or more doing this during the course of the day, but after about 30 to 40 customers have been in and connected (and left) the WiFi no longer accepts connections. There seems to be only two ways out of this

1. Restart the modem/router

2. Login to the modem and manually delete smartphone connections.

The problem seems to be that the modem/router "hangs on" to connections even after the customer has left. I have personally confirmed this by connecting my smartphone, then leaving the shop and going far enough away so that I am no longer connected, then turning the phone off. When I come back and login to the modem, my phone shows as a live connection even thought it is turned off.

I have both the 2.4 ghz and the 5 ghz enabled, and also have the Guest system enabled on both 2.4 and 5 i.e. there is a choice of four wifi connections that customers can choose from;

shop

shop-5

guest

guest-5

Does anyone know of a solution to this problem?

Is there a setting that will allow me to force connections to be dropped after a certain period of time with no activity?

Is there any way to increase the number of connections?

I have a second identical HG659, and I also have a Linksys X3500 (non-fibre). Is there any way that either of these can be used to expand the number of allowed connections?