I know we had issues with BT at the start to do with our FTTP installation,
..... and a hiccup with your router as well I seem to recall.
Well yeah, and I still am.
Fritz Box 4040 works great IPv4 + IPv6 all fine, but then the router starts dropping packets every 6 hours after X days.
The Linksys Router is all fine apart from no IPv6 due to BT uses DHCPv6 and that router doesn't support that, as for it dropping packets, not really been able to check that due to BT keep resetting the routers external IP (still hoping to get a static IP), once I notice it reset I will enable the BQM again to see.
I was thinking of flashing the firmware on Linksys, hopefully that will support it.
I also sent you a PM to-do with something else.