Reseting your networks settings can cure odd behavior. An example of odd behavior if the app logging in fine with Wifi but not cell data, or just the reverse. I have no idea why or what happened to your device, this is just something that has worked on the past.
Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you’ve used before.