If you can't or don't want to use the roam plugin for publishing for some reason - you can do publishing directly on the Roam Garden website.
In Roam Research, go to the three dots in the top right corner and click Export All
(or Export for a specific page).
Then select "JSON" in dropdown and click "Export All" button.
You'll get a zip file as a result - you need to unpack it, and then you can upload the resulting JSON file when prompted at https://roam.garden/app