A question about time series analysis with constraints

Hi, everyone

Recently, I encounter a problem about time series analysis with constraints.

Specifically, given the following data for n time points from t_1 to t_N:

feature: f_{t_1}, f_{t_2}, ..., f_{t_N}

label: l_{t_1}, l_{t_2}, ..., l_{t_N}

constraint: l_{t_n} in {1, 2, 3, 4, 5}, and l_{t_i} <= l_{t_j} if t_i < t_j

I want to construct a model to predict l_{t_{N + 1}} from the above data and f_{t_{N + 1}}.

Does someone know how to build a good model for this time series analysis? Or give me some clues and discusses.

Thanks in advance.