Set your zones based on 20 minute threshold using the British Cycling calculator.
https://www.britishcycling.org.uk/membership/article/20120925-Power-Calculator-0
Then set the zones manually on your device and/or strava, and memorise the numbers or paste them to your stem.
Then train. For fastest performance, sweet spot (boundary of 3/4) for 20 minutes times two. That will be very tiring but will show very fast results.
For recovery use zone 1. Use the inner ring and keep spinning with low force. Recovery rides should not be longer than an hour.
You can train in zone 2 (endurance) but will need to log the hrs and this is often not feasible.
For me, threshold at 47 is 170bpm. I can hold this for 20min, and the test is a LOT nicer and reproducible than maximum. See what yours is on a hard 30 minute effort.