I ran into your problem too where I find the screen resolution is locked (resolution select is grayed out), using a and a Razer Core V2 with a Vega 56, with currently latest windows 10 version. Does anyone have any ideas? Thanks in know this is a bit old, but for someone who just start using a egpu, I figure this could be useful so leaving my finding hopefully someone will find it useful. What I'm trying to find out is whether there is any way to change the resolution of the game when the iGPU is disabled, but I can't find any working method. Some users in other forums suggested changing the resolution using regedit, but the suggested folder to find the entries isn't there either.

Changing the config file doesn't affect the resolution in-game, even when changed to read-only.In-game, the only available option is 2560 x 1600.When I enable the iGPU and change the resolution, then disable it again, the resolution changes back to 2560 x 1600.In AMD Radeon Settings, the Display tab gives the message 'No displays are currently connected to AMD products.'.In Settings>System>Display, the resolution box is grayed out.

However, when I do that, the game's performance takes a huge hit because my resolution becomes 'locked' to 2560 x 1600 (I usually run games at 1280 x 800). I'm try to run Dishonored 2, but the game doesn't work with switchable graphics and only launches successfully when the iGPU is disabled.

Windows 10 resolution locked