I was having the same issue with a D5600 and iPhone (iOS 14), where the camera would connect to SnapBridge via bluetooth but the WiFi connection failed every time I tried to use remote photography.
What fixed it for me was going to the SnapBridge settings in my phone (Settings > scroll down to SnapBridge) and enabling "Local Network" access, which is disabled by default. Once I switched that on the remote photography immediately started working.