Thanks to both for the very informative replies, much appreciated.
With respect to the RPi (DSLStats) LAN connection:
It depends how you go about it. The easiest method is to have the modems LAN as a static IP address outside the routers DHCP range and plug one of the LAN ports into a router LAN port.
A more complicated method could use the single cable in the WAN port and some fiddling on the router to allow it to talk to both the WAN and the LAN side of the modem down the single cable, but personally I'm not fond of that as I'm not convinced some LAN traffic can't leak out the WAN side that way, at the very least wasting a bit of bandwidth.
Yes,will likely opt (initially anyway) for the 2 cable connection mode, I plan to give the new router the existing ZyXEL modem/router's IP address and allocate a new IP address to the (now) ZyXEL modem.
I always allocate fixed IP addresses by MAC's to all my LAN devices and plan to do the same on the Asus router.
I do the same with my Wi-Fi connections by MAC filtering.
Just to be absolutely clear, DSLStats will still collect it's data from the ZyXEL (now modem) via the Rpi ?
Questions 2 & 3 very clearly and comprehensively answered, thank you.
Still awaiting delivery of my Asus RT-AC68U from Amazon, I resolutely refuse to be forced/tricked into signing up to Prime and I'am periodically "punished" for doing so, this item is one such instance, 10 days proposed delivery on an in stock item.
Can't really blame the current Covid-19 restrictions as my recent Amazon orders (non Prime, free postage) have been remarkably fast.
Anyway, another topic not relevant to this post which has been well aired previously.