I have observed- on different phones and different locations (data set of 6 people), Fido app (My Account) doesn't work when the mobile data is on. The app works well on WiFi. I believe this is to deter people from using the Data Bytes.
Current App version: 3.80 (faced problems on 3.7, 3.7.1 and 3.7.2)
Phone: iPhone X (version ios 12.1.4)