I have a new Hue hub I got before Christmas ’18. I set it up (replacing an old round hub) and added more bulbs, some switches etc. and it was working fine. My router/network is a Linksys Velop mesh with 2 nodes.
The Hue was working fine. I had a power problem and the Linksys reset itself several months ago. The Linksys rebooted just fine. Since then I cannot connect with my iPhones to the hub using the Hue app. But the Hub still works through HomeKit. So I can use the Home app to turn lights off, etc. or the HomePod to turn lights on and off. (And the phones can connect to both local and internet connected services otherwise). The Hue hub is wired into a switch that is wired to the main Velop node and all other devices on the switch work fine.
Going into the Linksys management app, the Hub shows as not connected. I reboot the hub and nothing changes. I am not sure what to try further and why HomeKit works with it still.