It could be your USB HDD is powered from the USB port and is drawing too much current. I have a two external 2.5" external drives and one will power fine from a USB HUB port but the other won't work....

Try turning off (then back on) all the other network hubs you have as if they are switches they may have stored incorrect routing information. Also check when Windows say the new network it did not...

In the Advanced DSL Settings have you tried to enable Bit Swap and SRA?

Mine has been working rock solid for the last 6 months (BT Internet). It never drops the connection. As mentioned update the firmware but also reset the router to factory default sett8ngs, especially...

You need to select Wireless router mode as you will be connecting the TP-Link to your fibre box via an Ethernet cable so will not be using the TP-Link built in DSL modem.

It never hurts to reset the router to factory and set it up again in case a firmware update upset something. Of course it could also be an issue with your line or ISP.