Before and besides the actual configuration, you need to understand whether the WapAx is a suitable device in your use case.
Unlike most Mikrotik SoHo devices, the WapAx has a pretty much directional antenna(s) and emissions, so to use it as a repeater, BOTH the “incoming signal” and the “covered area” should lie in the same 90-120° angle, see:
If this condition is met, the hap ax lite has only a 2.4 GHz radio, so you will need to set the Wap Ax 2.4 GHz as client (station in Mikrotik jargon) and use the 5 Ghz one as AP for the wifi coverage of the sauna building.