matt17: Ok - how do I set a static IP for the Huawei? Does turning off DHCP in the Huawei have this effect?
When I check my IP address is the iMac's 'Network' preference pane, it does seem to be moving around. It's currently at 192.168.1.104 - I'm sure the other day it was .112.
Yup, thats how DHCP works.
matt17: Also - how do I check the IP addresses of the Time Capsule? I think the D-Link seems static at 192.168.1.1 as if I enter that into a browser, it brings up the D-Link's login page.
The internet feed from the D-Link currently enters the Huawei (via ethernet cable) - can I simply use a spare ethernet port on the Huawei and link it to the iMac using a cable?
Yes you can. But if the Huawei has the same IP address as the DLink it won't send traffic on to the DLink. There will be a "LAN" or similar local network setting in the Huawei where you enter IP address of the router (eg 192.168.1.251); subnet mask (255.255.255.0) and choices for local address allocation (DHCP off — or perhaps DHCP relay from the DLink).
With DHCP off you will not be able to access the Huawei for setup etc unless its already connected to the DLink because the DLink is the only router that tells everything how to use the network.