If the LG TV is visible to other devices on the network, such as the PC, you should see it within "Media Devices" under "Network" in the Windows File Explorer.
If you have an MPEG-4 (.mp4) or Audio Video Interleave (.avi) file on the PC, you can right-click on video file, then choose "Show more options," and then choose "Cast to Device" at which point you should see the LG TV as a device to which you can stream the video.
You should then see a video control window appear with the message "Contacting network device..."
The first time you attempt to cast to the LG TV from the PC, you may be asked on the TV to confirm you wish to allow the connection, so you will need to respond using the TV remote control. The video should then start playing on the TV with the audio coming from the TV's audio device.
Note: you can use the stop and pause controls on the Cast to Device window on the Windows PC to stop the video or pause it. If you use the stop control, the video will start from the beginning when you play it again. When I've used the pause control, which changes to a resume control when you pause the video, and then resumed playing of a movie, the video resumed, but not the sound, however.