xrdp is a virtual xserver running in software. nvidia-settings has to connect to the real xserver running on the nvidia gpu and the user has to have access to it.
So on another xserver you can’t use the gui, only cli, see
https://forums.developer.nvidia.com/t/tuning-nvidia-settings-over-ssh-error/60673/6?u=generix
Addendum: maybe the gui can be used by using the -c option, IDK.