@David-J 
did sky provide you with a different router?
 

from what i've worked out the tpplc mostly uses broadcasts & layer2 to communicate, since some devices don't even have an ip address. and it also seems to use an ethertype that's not all that common. (managed switches have a tendency to drop these).

my best guess would be that the router is either not forwarding broadcast to ethernet ff:ff:ff:ff:ff:ff , which would be strange. or is not forwarding to 00:b0:52:00:00:01, which is less strange since that's the mac atheros devices use in boot mode, or doesn't like the ethertype you're using.

i had a look at the pa4010 manual & from what i can deduce it doesn't seem  to have an ip stack, so you'll need tpplc to access it. otoh , how often do you need tpplc? if seldom could just replug your pc at that time.

if it's actually one of the above issues a cheap unmanaged switch in the router and your pc & tp pla adapter in that switch might solve it as well.