UPDATE:
Finally, I was able to get it to work. The answer was in this post:
askubuntu_com/questions/836448/2560x1440-resolution-on-16-04-and-ati-amd-radeon-hd4670-card/892091#892091
Bsically, the trick is to set the modeline using umc and not cvt nor gtf
in my case:
umc 2560 1080 54 --rbt
2560x1080x53.92 @ 59.743kHz
Modeline “2560x1080x53.92” 162.500000 2560 2608 2640 2720 1080 1083 1087 1108 +HSync -VSync
why 54 ?
According to this article: hwiki_archlinux_org/index.php/ATI#QHD_.2F_UHD_.2F_4k_support_over_HDMI_for_older_Radeon_cards
Older cards have their pixel clock limited to 165MHz for HDMI.
So the first number after Modeline “2560x1080x53.92” can’t be over 165 otherwise you get a black screen. An alternative, as described in the article, is to re compile the kernel and modify the radeon.hdmimhz parameter as posted in this other elstel_org/software/hunt-for-4K-UHD-2160p.html.en