I've resolved this three ways:
1. WAP/AP issue. This problem can happen when devices on a Wireless Access Point (WAP or AP) are isolated from the those devices using the router wifi. I hadn't noticed an AP isolation setting on the Ultra Hub but I guess it could be there.
2. Dual band issue. I have had that error message in the past with and without WAPs. If you don't have a WAP then I would check that both devices are using the same band i.e. both on 2.4GHz or both on 5.0GHz but not a mixture of bands.
3. Restart needed. Finally, I would restart all the devices on the network: router (Ultra Hub), Chromecast and phone.
Edit: I should have added that I've sometime I've had inexplicable ongoing problems with the various Chromecasts (gen 1 and II) in combination with routers from various ISPs including Spark. Sometimes the problem suddenly clears up and never recurs. At home I have Chromecasts connected by cable and WiFi but I also have one that I use almost every week to do presentations in other places. It's unusual to get that error because the Chromecast, TV and my phone/laptop are usually in the same room when they're being setup so different WAPs aren't an issue until I go to some other part of the building and lose remote control of the Chromecast.