How do I hook up lg smart tv using the analog coax directly from the cable box to the back of the tv?
Using a coaxial cable between box and television will only allow a SD (420p or worse) video signal and only mono., or stereo audio. You did not say whether your "smart tv" is just HD or 4K capable.
You really need an HDMI cable that can handle your new tv. If you try to do what you suggested you cannot watch better resolutions like 720p, 1080i, 1080p, and 4K. Also: surround sound systems for muti-speakers. You could have saved your money (cheaper tv) since using the coaxial cable cannot do any of the newer features of a "smart tv".