I've identified a problem with either the VR900v2 that is present in all available firmware including the latest 0.4.0 0.9.1 v0069.0 Build 171128 Rel.42667n.

The VR900v2 is very susceptable to bufferbloat, especially upload bloat.

Example: If you are on a slowish connection, in my case 14Mbps down and 1.5Mbps up, and try to upload a large file, the bufferbloat will get so bad the modem will start dropping packets and cause delays of up to 10sec. On top of this, it will get so bad it will disconnect and drop dsl sync completely.

My test: Ping www.google.com and see what a normal ms ping speed is. Then start uploading a file and watch the ping times ramp up to 10sec +.

Settigng QoS does fix this, but there is another bug in the QoS GUI. The GUI does not allow for decimal places. For example, if I want to set the max QoS for DSL upload speed to 1.5Mbps, I cant as the GUI does not allow decimal places and on the Kbps setting, it only allows up to 1000Kbps. It will give this error: 

"Error code: 4813 This number should be between 0 and 1000." Dear TP-Link..... FYI 1.5 is a number between 0 and 1000.... You want integers? Really?