Is it advisable to do a PhD at low rank university (~700 QS) if the advisor is well known in the field and is actively involved in research.
QS ratings cover the whole institute. But in many universities there are a few really good research groups weighed down by a lot of weaker ones. In the appropriate research field people know that and will give due esteem to them. If the advisor is good and the research group is good then you will (hopefully) do good work and get the glory - ignore the league tables.