Seems to be mobile 3g/4g5g/ coverage of mobile data when switching off WiFi to mobile data doesn’t work. I tried using my mobile phone as a personal hotspot and connecting again to the app and it then works, once off nothing works again.
Already give my answer in another thread but I will reply here to 
Got the same problem and finaly found a solution. Force your mobile phone APN to use Ipv4. By default my LTE force it to Ipv6 and it does not work with eufy app…
Please eufy try to manage it working with IPv6 Also de are in 2020
How do I do that I have an iPhone 11 pro