Forgot to mention, since you got a new Wi-Fi mesh router, best practices would say to make sure the smartthings hub is at least 3 m away from the Wi-Fi router. I know a lot of people will say that they have their Wi-Fi router right next to the SmartThings Hub and everything runs fine, and I’m happy for them, I’m just telling you best practices since you are having some problems.
If you are interested in reading more about these issues, start with post 11 in the following thread (i’ll link directly to that one), read it, then go back up to the top and read the whole thing. 