You say that BT works OK and that a Wireless hotspot also works for you.
The Bluetooth feature uses the same antenna as the WiFi and Wireless hotspots use WiFi to connect between the devices, so it would seem unusual that it is an antenna problem.
What is the WiFi signal level as shown in the WiFi settings connection area or by the WiFi signal bars when next to the router and when further away?
Try starting the phone in safe mode and check if it now connects to an AP. If it does then a downloaded app is the cause of the problem. The trick is to find out which one.
If you wish to check the antenna connection, here’s a link to the ifixit OnePlus 3 Hauptplatine austauschen guide. At step.11 you’ll find the antenna connection.
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