Hello @Jervis 

• You could plug powerline unit into the wall socket, and plug router to a power strip and somewhere further away from powerline device. 
• If its a VDSL modem, refer to instruction below and enable lower PLC-to-VDSL interference mode:

What does Lower PLC-to-VDSL interference Mode do on powerline adapters?

• Note that only one powerline  unit need connect to main router via Ethernet cable, another powerline unit is in a different location to extend network (do not connect both powerline units to router at the same time)

If still the same, may I know the model of your Talk Talk router or have a photo of label on router? How about the led status on router when issue happens. 

Hello   @Jervis 

Sorry for the late reply. If the router is down ,can you check if the LED status on router/ Modem including Power led / Internet Led/DSL, do the LED  look the same as they work fine. At the same time, it is recommended to reach router support and see if they meet any similar case or suggestions. You might need to do some advanced troubleshooting with the router support and see what happened before router is down. 

Make sure you have enabled this mode What does Lower PLC-to-VDSL interference Mode do on powerline adapters?

May I know the model & hardware version of your AV1300?  If it's a Wi-Fi kit, you could also turn off the DHCP for Wifi powerline unit as thread here. 

When reach router support, can you find the model of router, and what is default IP address of router? 

Did you get a new modem & a new router?