Based on your screenshot above, it looks like you’re potentially using the device running Arc as a mobile hotspot device - is this correct?
Do you experience the same login issue if the device is on Wifi, or over cellular data and not as a hotspot remote?
Do you experience the same login issue when using a different mobile browser? I’d ensure any popup blockers or third-party security on the iOS device are temporarily disabled to ensure you’re able to gain proper access to upstream services.