That fixes that, yes, but Roku is now evidently inspecting the host and/or referrer header to make sure that the request comes from the same network, and throwing 403 forbidden otherwise.
I'm using a website on another network to show the web gui, and although it is technically the local client doing the requests directly to 192.168.x.x, the Roku is somehow seeing the website address and throwing 403.
If I run the exact same html manually/directly from chrome instead of loading it from the website, it works properly.
This is due to another security feature: https://community.roku.com/t5/Roku-Developer-Program/External-Control-API-suddenly-returns-403-Forbidden/m-p/499344