As soon as I disconnect the Apple USB-C power cable the LG monitor immediately dies. I have replicated the problem several times, reliably. Once I connected the additional USB-C cable feeding power into the adapter it was then able to provide enough voltage to transmit a stable enough video signal across the HDMI cable to the LG monitor to allow it to work. The 'digipower' adapter does not receive enough power from the MBP to be able to push a strong enough HDMI video signal to the LG monitor. Eventually I tried plugging in my Apple provided USB-C power cable into the USB-C port on the 'digipower' adapter and the LG monitor suddenly came to life. When I did this my LG monitor quit working and nothing I found on this thread or any other trouble-shooting sites seemed to help. I then acquired a new, AUKEY, 10 port USB-hub to connect multiple external drives to my MBP. When I initially tried to connect the monitor, it took some messing around with the connections but I eventually got the display to work and things were fine for the first two weeks. The USB hub has an HDMI out which was my preferred method of connecting to the monitor. I purchased a 'digipower' 7-port direct connect USB-C hub to use with the MBP. I wanted to connect my new MBP to an external monitor (LG 27' UHD 4K, Model 27UK850). I recently purchased a 2020, 16-inch MBP to replace a 2008 16-inch MBP.

Mac not detecting usb c device