I think it's same as this guy's problem: https://askubuntu.com/questions/809858/amdgpu-pro-secondary-screens-vga-w-dvi-doesnt-work-but-hdmi-does. Two outputs are shown, DVI-D-0 and DVI-I-1 but no picture on DVI-I-1. This happens in every version of amdgpu open source driver. AMDGPU-PRO driver detects second monitor as DVI-D-1 and it works.
This may be just a default kernel problem. I installed these packages (firmware and UBUNTU 4.13.11+ .deb's) https://github.com/M-Bab/linux-kernel-amdgpu-binaries and monitor is detected as DVI-D-1, as it should be.
On Ubuntu 17.10
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