Speed drops dramatically

I’m cycling along at 35kph then suddenly I drop to like 3kph, I’m new to Zwift using a dumb trainer, Cycleops Fluid 2 with Speed Sensors on the rear hub, also I can only connect via 4g as using outside so no WiFi connection.
Does Zwift assume my speed and if I go uphill it will automatically drop my speed based on Watts output or something or am I losing connection even though it doesn’t say so
All help welcome

Hi @David_Turner5, please read this thread about how your speed is determined. If you are on a hill you will need to put out more power to hold a steady speed.