Same happens in my case. I can use Bixby using my home router which is an older Linksys model. But my office router is RT-68u Asus, with which I cannot use Bixby on my S8. I dont know how to solve connection problem with Asus router.
I don't understand how it can be more of a problem for newer users than the older subscribers. Every user downloads the same app from Samsung which seems to me should mean that no one persons app should run any different than the next persons.