You have a device (e.g., some IoT gadget such as an EZviz camera) that can connect to a network (e.g., via WiFi). But you do not have access to the administration (e.g., back end) of the router itself. You want to learn the IoT device's MAC address (e.g., for a third party that can grant the device WiFi access). If you administered the router, you could find the MAC address of devices that could connect to the router. You tried looking at the labels of the physical IoT device and inspecting the paperwork (documentation and/or instruction manual) that came with the device. You still do not know its MAC address. Like all IoT devices, it can connect to a TCP/IP network (e.g., the internet).
Solution with an Android tablet/phone
Connect the Android tablet/phone to the same network the device in question is connected to. On your Android tablet/phone, install Fing from the Google Play Store. Look at the MAC addresses of the devices that are on the network in Fing. Connect the device to the network and refresh Fing (e.g., by clicking a semi-circle arrow in the upper right-hand corner). Find the device that recently connected to the network.
Solution with a Windows computer
Connect the Android tablet/phone to the same network the device in question is connected to. On your Android tablet/phone, install Find MAC Address (or if you need a business edition, try this link) by Lizard Systems. Search for a range of IP addresses on the network to find the device in question. Find the device that recently connected to the network. If you are not sure about the device in question, narrow your range or do A/B testing. For A/B testing turn the device off then do a scan with the Lizard Systems' Find Mac Address for an "A" test; turn the device on then do the scan via the Lizard Systems' Find Mac Address for a "B" test).