It took me about six hours to get the thing to work, and I was just about to give up. Fortunately some clues from a post on Geekzone I discovered through Google made all the difference. But even so it still took a long time and some distinctly different things to what was set out in that post to get it working. Nonetheless Geekzone was very helpful, so I signed up afterwards to submit this message in the hope someone else can benefit from my experience.
The solution was:
Both units (Linksys router and Dynalink modem) originally defaulted to the same IP address of 192.168.1.1. I had to leave the DHCP servers enabled on both devices (as per defaults but contrary to most online advice to only keep one) but change the Linksys router IP address to a different subnet. I changed the Linksys IP address to 192.168.0.1. Just changing the Linksys to a different IP address within the same subnet didn't do the trick.
I then set the DHCP address allocation range on the Linksys to fit its new IP address. So the Linksys DHCP allocates between IP addresses 192.168.0.100 and 192.168.0.254.
I had to set the Dynalink RTA1320 modem connection type to PPPoA, disable NAT (enabled by default), and enable PPP IP extension (disabled by default).
Finally in the Linksys router I set internet connection type to DHCP - Automatic Configuration.