Is there some way to play youtube on WDTV Live please? Some custom Firmware or a browser hidden app? I read somewhere for UMSP and plugins for youtube but i don’t know how to manage with that 
Yes, there is:
Take your android device
Install bubbleUpnp, set it up to use wdtvlive as target
Open YouTube official app
Find desired video
Press play, then pause
Press share button in YouTube app and share the link to bubbleUpnp in the dialogue opened
You are done!
P.s. if bubbleUpnp says format unsupported uncheck the option in bubbleUpnp which tries to check format before play
