The Neet box that I originally bought says it supports 4k up to 60khz, but interestingly does NOT say that it goes all the way to 18gps. Maybe that was the problem, but it's a pity that Neet did not themselves mention this when I was talking to them. They seem to have a new product that does support 18 gps, but it doesn't have the flexibility of the one I originally bought, and will only output 2 x 2160p, so wouldn't be of any use to the member who wanted 1 x 2160p, 1 x 1080p.
Would I recommend the other box I bought? I did have some issues when I first got it, trying to get both screens active, but it seems to have settled down and is behaving itself now. My setup is either one display or the other, rarely both together (although they are in different rooms). The picture quality is excellent (as it should be!). I tried it with another 4k source (a PC) and it certainly worked at 2160p 60khz.