Magnetic trainers

I am using a magnetic trainer with speed, cadence and heart rate monitor. when i did a ramp test my number were very low and i assumed it because of my equipment. in order to push my self i up my my ftp but still believe my number would be much higher igf my equipment is right. I know with ant + devices sometimes the number might be off and that’s why you need an extension usb cable( which i don’t have). it only the cadence meter that connects to ant + the rest is bluethooth. I am looking for help and love somerecomendation in order to put more correct or closer to the correct numbers when i workout. with the low number(which in my mind is wrong numbers) i do work out real hard. so please guide me on what i am doing wrong.