@David-TP 

In the ongoing saga (starting to sound a bit like Days of Our Lives), I bought a new modem, on recommendation from TPG. It arrived this morning (a TP-Link VX420-G2v AX5400) and it is now up and running. I also have an old PC, with an ethernet port, set up next to it so I can test the raw speed. First thing I did of course, and it's delivering 770Mbps down the cat6 cable. The X86 is still running way slower at about 76MBps with the PC connected to it by wifi. I also tried the modem -> cat6 -> X68 -> cat6 -> PC test again, and the X68 is delivering 79Mbps out of the second ethernet port. This seems odd to me. The specs say that the ports are gigabit ports. Could the X86 be faulty? I bought it from an Ebay seller, so it might have been used/refurbished. Is there any way I can test for sure?

Thanks,

Ian