The technical terms for the network you are creating are "Mess" or "Frankenstein"!
Set the TP-Link device to bridge/modem.
Reset the SH2 to factory defaults.
Set the SH2 to use a LAN port as the WAN port (it's in the menus somewhere).
Connect the TP-Link to Master Socket and connect a cable from the correct socket on the SH2 to a LAN socket on the TP-Link.
This should work, but it should also create more lag/BufferBloat than using the SH2 directly - you are adding an extra device that buffers data.
If you use just the TP-Link modem/router you'll need it's WAN configuration to be in PPPoE mode, VLAN:101, username bthomehub@btbroadband.com and Password: BT
That should work though you'll lose Digital Voice.
You can then try the QoS options on the TP-Link. The Bufferbloat tests though are artificial, they constantly max out the connection and look for buffering, not every connection is going to get top scores every time, and if your games aren't maxing out the connection it may all be completely irrelevant! As for lag jitter of 1-2 ms, that's par for the course, we all get some jitter!
*My own router reports NTP drift in nanoseconds, there are people on the related forums who worry about drifts of 5+ nanoseconds, that's 0.000000005 of a second (it seriously not even close to a problem)! Welcome to reality!
I only learn by making mistakes and owning up to them - boy do I learn a lot!
