How to add Kenward-Roger approximation in a LMM?

General
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I am comparing a series of 17 different LMMs and I am getting the warning message "boundary (singular) fit: see help('isSingular')" that could be related to low sample size. I was told that I could add Kenward-Roger approximation in the models to account for low sample size. So I am trying to find out how to best do that. Anybody can help me?

Is it always a comparison between large and small models? If yes, am I still able to run it since I am comparing 17 models?

Here is just an example of one of the models I am running:
oc <- lmer(Cortisol ~ Month + Year + Local + Age + Status + ΣPFAS + (1|ID), data = model)

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