Can you get the Fenix 5 plus to record speed and distance based on power and cadence when using a non smart turbo trainer?

you can't directly get to speed/distance based on power or cadence.  speed/distance is a little irrelevant inside, but you'd need a speed sensor something like the the Garmin Speed Sensor 2, but anything would do.

I take it your power meter doesn't send speed? Which power meter?

You can't translate power to speed without knowing or making assumptions about numerous factors such as gradient, air resistance, rolling resistance, rider weight. A power meter neither knows nor cares about these things.

You could generate a ridiculous speed for little power by lowering resistance on a trainer (such as a spin bike) to near zero, or make it almost impossible to move, even with colossal  power, by turning resistance to the max.

Unless you have simulation software to replicate an actual environment, or some sort of established parameters for resistance, coupled with a conventional speed sensor you're really on a hiding to nothing but nonsense.

Put it this way - if a vehicle is making 100 BHP at 4,000 RPM, how fast is it going? There is insufficient data to answer the question.

And back to the bike - you might make 200W at 60 RPM, 70 RPM, 80 RPM, 90 RPM, 100 RPM. What should your speed be?

If it is a wheel on trainer then you will need a speed sensor on the wheel with the wheel size properly set in the sensor settings to get speed and distance recorded.  From your description it does not sound like you have a direct drive trainer as those would send speed, cadence, and power which would have provided what you are looking for.