DIAGNOSIS The new point is too far from the primary router or another point, or there is significant physical obstruction (like metal or masonry) between them. The signal strength is below the threshold for a stable mesh backhaul.
FIX Reposition the point. Move it closer to the primary router, ideally within one room and with a clear line of sight. Complete the setup at this closer location. After setup is successful, you can power it down, move it to your desired final location (but no more than one major wall away from another unit), and plug it back in.
IF THAT DIDN'T WORK The weak connection warning may persist due to Wi-Fi congestion. Temporarily disconnect other nearby points during the setup of the new unit to reduce wireless contention. Set up the new point first, then reconnect the others.
HEADS UP The Nest Wifi Pro uses a dedicated 6GHz band for wireless backhaul between units. If you are using the Ethernet port on a point for backhaul, you must connect it via cable before starting setup for the app to recognize the wired topology.