Beryl ax router dies when I USB tether with 15W cable

I have a Beryl ax router that works fine for usb tethering to my moto edge 30 pro when I use an old smaller cable but when I use a 15W cable (both USBa to USBc) all sorts of weird things start happening. It works at first then the modem dies eventually and then I get things like ‘charging very slowly' and it won't USB tether again and basically stops working. I am also not using the standard power adaptor I have USBc to USBc connected to a wall socket that says PD 20W. I need a longer cable hence why I am not using the old cable that works. Any ideas how to fix this?

Cheers Toby

Hi

The Beryl AX router does not support the PD protocol.
For proper functionality, please use the included 5V/3A power adapter.

Please also ensure the USB cable is standard and has not been modified to alter pin definitions.
A cable rated for 15W is acceptable, but confirming compatibility is advised.