Make sure that you have the service running with
services.resilio.enable = true;
Also, there are other configuations you can look for in the NixOS search page like
services.resilio.enableWebUI = true; # To run the WebUI
services.resilio.httpListenAddr = "127.0.0.1"; # To bind the WebUI to only localhost
services.resilio.httpListenPort = 9999; # To specify a different port