If you need to get XRDP to work on NixOS with the xfce desktop environment, all you need to add to your configuration.nix is:

/etc/nixos/configuration.nix

services.xrdp.enable = true;
services.xrdp.defaultWindowManager = "xfce4-session";
services.xrdp.openFirewall = true;

You may want to create a seperate user for xRDP (Actually, you will, as you will be unable to log in to xrdp if signed in locally to your account, and vice versa).

An example for a user account just for xRDP would be alex-xrdp or xrdp-session. Whatever floats your goat.