@Marbo 

There's no option to turn on/off the RE650 remotely.

What you can do is to :

1. Use the Advanced Settings=>Power Schedule to turn OFF the RE650 on a scheduled basis.

2. Use the Advanced Settings=>Access Control (empty whitelist) to restrict any device to connect to RE650 when you decide.

3. Disable Wireless=>Extended Network (both bands) when you decide to restrict any device to be able to connect to the RE650.

You can have a look at RE650 v2 emulator here.

@Marbo I'd like to chime in and add some additional information. 

RE650 V2 is one of the models that supporting Tp-Link Cloud service, and this means this model can be remotely managed by Tp-Link Tether app after device has bound to your tp-link ID. 

For more details, please refer to UG Chapter 4 Tp-Link Cloud service. While you may not able to power it on off on this app, the app has the options to reboot or reset device on the app, while power schedule need to set up on the web page. 

https://www.tp-link.com/en/cloud-service/product-list/#range-extenders