Process ordinal data as a function of time in R

I try to treat ordinal data as a function of time in order to analyze if there is a significant difference between two different modalities.

My dataset consists of 30 readings taken each week in two different modalities. The ordinal data are scores given to individuals according to the abundance of aphids found on them (absence, a little, a lot).

I performed a linear regression using the polr function but time is taken into account here as a fixed factor.

predict(e,n4,type = "probs")
exp(cbind(OR = coef(e), confint(e)))

How can I correctly interpret such a data set ?