problem connecting router to ethernet modem

[#1]

I am trying to connect a Dlink Dir-815 wireless (but modemless) router to a Dlink 320B ethernet modem.
Everything appears to be fine. The properties page on the router shows everything is working and connected - both on the Wan and Lan sides. The green light on the router shows it is detecting the internet. The router is set to connect via DHCP - and that is working at least when you look at the router interface.
But nothing connected to the router either via ethernet or wifi can see the internet. To post this, I've had to connect via ethernet direct to the modem - cutting out the router.
Any suggestions?

[#2]

Doesn't your use of an Ethernet modem mean that you're not trying to connect to Plusnet?  AFAIK in the UK only Virgin Cable customers use an ethernet modem and that is provided by Virgin.
With a Virgin cable modem you have to power down the modem each time you connect it to a different device so I imagine if you tried to insert the router without doing that it might not work.     

[#3]

@rr, the DSL-320B is an ADSL2 modem so it should be fine on PlusNet.
@niggle, I believe you will need to set the modem into Bridged IP VC-MUX mode and then set your Dir-855 into pppoe and enter your PlusNet login details into that. The problem you have in the (assumed) default configuration is that both the modem and the router are doing NAT and the modem LAN subnet is the same as the router subnet ( 192.168.1.x ) and therefore the router will not route to the internet via the mdoem. Using bridge mode on the modem will elimate NAT on the mdoem and pass the public IP straight through to the router.

[#4]

@MisterW That sounds a promising line of attack. I'll report back when I'v had a chance to have another go. Thanks for such a rapid and helpful response.
And @rr the ethernet modem is fine. That connects to Plusnet with no difficulty. It's ADSL in and ethernet out.
Indeed I think it used to be supplied by plusnet as an option for those who didn't need wifi.

[#5]

MisterW's plan has worked a treat. Here's a summary for anyone else with a similar problem.
Connect your PC to the modem
Set the modem up for bridged access (ignoring the bit that says you should only do this if told by your ISP)
Then choose bridged VC-mux.
Now disconnect your PC from the modem, connect your router to the modem and hook that up to your PC.
Choose pppoe and fill in your user name etc (as you would for a modem)
It should then all work!
I'm very grateful for the advice.
I was trying every possible setting on the router, rather than attacking the modem as I was getting a perfectly good net connection via ethernet.

[#6]

Nice one MisterW, done in a true community spirit!

[#7]

No probs. Happy to help where I can.