Hello @jens,
Welcome to the Peplink Forum.
Have a look at the reply to @Michael234 on this previous post, shown are images on where to find what Wi-Fi frequency/standard your clients are using.
This should help you know what clients are connected to with your Wi-Fi.
As to why you are not getting the speeds you expect, first see what your devices are connecting on as it may give a clue.
Happy to Help,
Marcus 
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