Teradek Cube 255
This is a fantastic device that turns up to 4 WiFi devices (smart phones, tablets, laptops, etc) into wireless monitors. The Cube uses Wi-Fi to connect to an existing Wi-Fi network, then uses the network to wirelessly stream to any Wi-Fi device and then live monitor with it. You can even stream it online and view it remotely if you want to. I use it on most of my sets, I usually give an iPad to the director, have the producer connect to it with their laptop, have the boom op use an arm band with their phone to monitor frame and if we're shooting 2 cameras and I'm also oping I'll attach my phone to A-cam with the B-cam feed so I can monitor both cameras simultaneously. If you're using multiple cameras you can also get multiple cubes and monitor up to 4 cameras on a quad display on all of your devices. And the delay is very minimal , usually 1/3 of a second.
For power it can connect to p-tap, bullet, or AC.
If you want something newer, more dependable, with less delay and allows for up to 10 devices, I recommend my new Teradek Serv Pro: https://www.sharegrid.com/losangeles/l/83644?type=rent