If you use hotspot, then you would have records of who is using it… but if you provide open wifi then how would you know “who” is using it? When you say “what devices”… are you asking what devices the clients are using??? If so, then there is no way for a router to know what kind of device you have. It’s all mac addresses and IP addresses. If you are using a hotspot, you might be able to grab user-agents on the http… but otherwise all you have is mac-addresses which you can lookup to indirectly find device types sometimes (or network card manufacture).