@punchtownparry 

Hello, thanks for your requesting, we will do some troubleshooting first.

Who is your ISP and what is the bandwidth you paid? 

When speed slows down to 15Mbps, please click here to verify the wireless negotiation speed of your laptop.

And please connect your computer to the RE650 by wired cable and test the speed  you can get.

In the meantime, next to the RE650, with computer connected to the 5GHz of the main router directly, what is the speed you can get? 

Please ensure that the firmware of RE650 is the latest one. And try to move it closer  to the main router to give it a go. 

Best regards. 

@punchtownparry 

Thanks for your reply, from the statistics provided, the speed seems fine compared to wireless speed. The speed is affected by the distance, main router, obstacles and so on. 

You can test the wired speed from the RE650 when the wireless speed shows normal.

While we still need the wireless negotiation speed of your computer with it connected to the RE650 when the speed slows down. 

And in the meantime, next to the RE650, please connect your computer to the main router's wifi and test the speed you can get. 

Besides, please ensure that the firmware of RE650 is up to date, and you can customize the wireless settings of main router. Like wireless channel and channel width.