I am having the same issue with my Windows 10 on a private network at home.
Windows 10 Version 1803 (OS Build 17134.407)
ROKU Model 4660X - Roku Ultra, Software Version 9.0.0 - Build 4072-46
My Windows 10 Computer is a Lenova Laptop connected to the Xfinity Wifi on the 5g connection
The Roku is also connected to the same Xfinity WiFi 5g connection
The WiFi Cable Modem is an ARRIS Group, Inc. Model TG1682G
I am able to remote control my Roku Ultra, but I cannot sign in to the Roku Account from my Windows 10 Roku App, but I can sign in using my Androd Tablet Roku App