20 Granted. But they were broken off because of unbelief, and you stand by faith. Do not be arrogant, but tremble.

Read Romans 11:20 Using Other Translations

Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear:
That is true. They were broken off because of their unbelief, but you stand fast through faith. So do not become proud, but fear.
Yes, but remember—those branches were broken off because they didn’t believe in Christ, and you are there because you do believe. So don’t think highly of yourself, but fear what could happen.