You could have shitty routers. I use unifi access points, make sure I set the channels so I don’t have a lot of interference with any nearby wireless networks, and I should be able to handle a few hundred devices at once. You could also have a small DHCP scope that limits the number of devices on the whole network.
They have an app and they do connect up to it. But they can be put on a vlan and null routed to only work locally.