gkatev CEC is probably something that will never work in all cases. That being said, H3 uses SW implementation which should work with majority of TVs. I forgot details, but IIRC address f.f.f.f means CEC is not supported. Make sure it's enabled on your TV (common issue). Another possible reason would be AVR (if you have it). Try connecting directly first. At the end, you can also try different HDMI cable. Some cables may not have CEC line connected.
okabekudo It could be voltage issue. That can be easily checked with voltmeter. Note that your box is not supported by Linux nor U-Boot and I don't have it, so you're pretty much on your own to figure this out. Strange issues are not uncommon for TV boxes due to cost optimizations. That's the main reason why Allwinner port of LE supports mostly SBCs and only 3 TV boxes which we have and test images on them.
U-Boot development process is similar to Linux, which excludes any fancy stuff like issues on github. In fact, U-Boot github page is unofficial and just clone of official gitlab portal. Issues should be reported on mailing list with CCing respective maintainers.
Uphantom88 USB drive is not supported boot media. Only SD card, eMMC, NAND and SPI NOR. LE supports only SD card and eMMC. You can open box up and check which storage is used. If it doesn't have SD card nor eMMC, you can forget doing anything with it. Even with eMMC only, it would be pretty tedious to boot LE over FEL (USB protocol used for "rescue"). As I said before, there is a reason why only few TV boxes are supported.