LG has disabled this feature by software.
As you can see in the attached screen, the smartphone detect a displayport adapter, but the tv out is disabled via software.
I have been researching into the kernel source, and I have discovered how they have disabled this feature, I have uploaded the hdmi_Kconfig file from LG v30, and Samsung Galaxy S9, both of them support HDMI out, and LG G7.
There you can see waht I have explained.
If it would have some developeres interested in this device, they could take back this feture to LG G7.