I am looking for the drivers for the above processor.
Currently, I can use the main laptop screen with the default driver, but I cannot get a monitor attached to the HMDI port to work.
Any ideas on how to fix this issue?
I don't know why AMD do not offer closed source drivers for neither Ryzen APUs, regardless for desktop nor mobile or using Ubuntu Rolling Release as 19.04 nor LTS as 18.04 but that's how things are.
Let me advice you report it via Launchpad , I guess that Ubuntu Developers Community are in frequent contact with the AMD Developers for Linux, I can't confirm it but you'll probably find better support than here.
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