I suggest you can contact with TP-Link technical support to solve this problem, I updated a beta firmware provided by them two weeks ago, everything works well now.

@woozle Yes, you're a professional. But switch the channel back to the original channel is not necessary according to the DFS standard, it depends on the manufacturer. If the router has this...

Did you enable high device priority feature on the Tether APP?

Did you enable High device priority or QoS feature on the Tether APP? I tried to disable these two settings on the Tether APP, my speed restored to normal.

Maybe you need to set up your internet connection?

I suggest you can try to use wireless router modem, and use EWAN port to connect your VR600 to fiber modem by cable. If you want change the operation mode, you can change it from Advanced->operation...