Unity Mirror — WebGL Hosting on AWS EC2
 configuration just for testing.
So I intentionally hosted over HTTP — but it turned out WebGL builds couldn't be hosted on the server that way.
The fix was surprisingly simple: host on a non-WebGL platform instead.
Once I joined as a client, things went smoothly :)
By the way, Mirror isn't an official Unity tool — it's a free community-built networking solution.
If Photon feels too expensive, Mirror is worth considering.
Or if you're thinking about performance at scale — hundreds or thousands of concurrent connections — MLAPI is a solid option to explore.