Diagnosis
The plastic locking tabs on the dust port or the router's housing are worn or broken. The fit becomes loose.
How to Fix It
Inspect the two locking tabs on the inside of the dust port. If they are rounded or cracked, replace the dust port (part of base assembly). If they look okay, check the corresponding slots on the router housing for debris. Clean them out.
If That Didn't Work
Wrap a single layer of electrical tape around the router's port where the dust port clips on. This will increase the friction fit. Do not overdo it or you won't get it on.
Heads Up
A flying dust port is a distraction and hazard. If it won't stay, remove it and use external dust collection until you fix it. Don't rely on a loose connection.