Hope I'm not stepping on any toes answering this, but this is an issue I'm rather familiar with. Switching your phone to airplane mode/disabling cellular connectivity is the best way to diagnose this, and fix it temporarily, though I understand a bit tedious to do every time.
I find this is most commonly the phone "turning down" the wifi source since it's not a true wifi connection. For Apple phones, going into your cellular settings, and turning off the "Wifi Assist" feature can permanently resolve this, and for Androids, disabling the "Smart Network Switch" setting will resolve this too.
Here's hoping that the Viofo app overhaul addresses it, though we'll have to wait and see.