Speed Sensor on 29er

Hey team, just bought a Saris Mag (dumb) trainer with a Saris speed sensor and started my Zwift membership. Speed sensor seems to be tracking my watts fine but shows me going max of 2 mph. Riding a 29er MTB for reference. Any ideas on how to get an accurate speed reading ? I’m pedaling for 30 min and it’s only showing I go 2-4 miles