Not sure if this has changed since last app update but one of the reasons I love Hue Essentials app is because it didn’t check for a connected Wi-Fi connection while the original Hue app always did, and which I hated for that reason.
I often use the app over a 4g/5g mobile internet connection while connecting to a VPN server I have set up at home, always worked perfectly!
But now since recently I cannot do that anymore because the app gives me the message: “Unable to connect. The bridge cannot be reached. Please check your Internet connection”.
The funny part is that at the same time the Bridge says it is connected and I can also dim / brighten lights over the VPN with the 4g/5g mobile connection. But getting the “Unable to connect” message while doing other functions like enable / disable lights etc. If I connect to a random Wi-Fi somewhere then all works fine over the VPN… So the app just wants to be connected to some random Wi-Fi for full functionality which wasn’t the case before 
If it is the case that this Wi-Fi check has been implemented recently then the app has become useless for me, just like the original Hue app. Maybe make a setting where advanced users like me can disable the Wi-Fi check so the app can ignore it.