It's an ASUS RT-AC66U_B1 with the merlin firmwire. I wondered that as well, particularly as the packet loss isn't as bad on the samknows box, presumably that is pinging outwards to a server rather than pinging in? I'm not really sure how it works though. I'm hoping to find some time today to swap the router out and get an answer one way or another, I'm also wondering whether it's worth the effort of swapping out the modem as well, also will I still get the dropouts with the Zyxel VMG1312-B10A now the SNR has been raised. Also, although the TP-Link TD-W9970 is showing as not having any resynchs I'm beginning to suspect that isn't the case as I've noticed that the external iP has changed several times and the router occasionally drops connection to the modem.
So many unknowns atm.