That HDMI 5" screen works perfect with Raspeberry Pi using ubuntu mate too. Nevertheless, the Bananapi M2U works without no issues with my TV on 1080 resolution, but i can not find the way to use my small HDMI screen.
This is the worst user forum ever. Here nobody never answer, even if in the past they had the same issue and not to mention the Banana pi tech support becuase it is the worst too. I wrote several times asking for a solution and they always told me that they haven’t tested that 5 inch LCD screen while the problem is not the LCD by itself, it is a problem of legacy kernel 3.4 or 3.10 that banana uses for their boards.
What board do you have? my solution was to move to Armbian nightly image with 4.13 kernel, and that fixed the LCD without doing nothing. That 4.13 kernel support all the boards that are allwinner H3 based, but board with R40 like BPI-M2 Ultra, will be supported on next 4.15 kernel that will be release by the end of the month or beginning of february.
Armbian is lighter and so much faster than all the banana pi images, even as the nightly is based on ubuntu too.
PD: nightly means that is a beta version (not considered stable), but the true is that it is perfectly stable.