I’m having the same issue. I changed my modem on Wednesday and my Hue lights stopped being controlled by my SmartThings mode changes.
I’ve used ST and Hue for years, and can still control Hue lights via the Hue app.
I deleted Hue from my Linked Services and tried re-adding. I tried first option with the ST Hub. That didn’t work.
I tried the second option w/o ST hub. That worked better, but when I logged into my ST account, the first message I got was that the connection failed. I refreshed the page and got a message that it connected successfully.
I do not see my Hue lights in my ST app at all though. I called Hue and they said the round V1 version of their hub (which I have) is no longer supported for 3rd party internet functions. They said this change occurred in August, however my ST/Hue integrations were working fine Wednesday morning.
Again, the only change I made in my system was an upgrade to my modem. My router and other connections were all the same.
I called ST 800#, however they didn’t know how to fix this and put in a support ticket to engineer, but let me know that I should expect an extremely delayed response.
Is there something else I can try?