Why do video calls freeze when my WiFi signal is full?

My phone shows full WiFi signal in the bedroom, but video calls freeze there several times a night. The same phone usually works near the main router, and there is a mesh node near the bedroom. I cannot tell whether the bars only mean the phone hears the node while the node itself has a poor connection.

What is the first useful test to separate a mesh or WiFi problem from an internet problem? I would rather change one thing at a time than buy another node or reset the whole network.