How do I handle self employment reporting when a 2020 1099-NEC includes income that I did not receive until 2021, and I'm running my business on a cash basis?

I was paid monthly on a retainer as an independent contractor. Although they cut checks for November and December 2020 and included both on their 2020 1099-NEC, I did not receive either of them until January 2021. On a cash basis, should these two payments be reported on my 2020 taxes, or 2021 taxes?