The effective interest rate should be calculated and then converted to the monthly rate:

$$ \left( 1 + \frac{i_a}{n_a} \right)^{n_a} = \left( 1 + \frac{i_b}{n_b} \right)^{n_b} = \left( 1 + \frac{i_{\text{annual}} }{1} \right)^{1} $$

https://www.investopedia.com/terms/e/effectiveinterest.asp