LAN on Thomson 585

[#1]

I have just replaced a Netgear DG834 router that was providing internet access and a LAN with a Plusnet Thomson 585 v7.  I have done all of the obvious setup tasks and the internet access works fine but my LAN has disappeared.  None of my PCs will see any of the others even though the Thomson reports the PCs on both the ethernet and the WiFi connections.  If I go back to the Netgear, I get my LAN back.  I have tried various reset and power cycle options - is there anything obvious that I should have done?

[#2]

Have you checked your firewall settings.
When I changed routers my firewall saw it as a new connection and I had to designate it as local and not public

[#3]

I am really impressed by the speed of your response - notwithstanding it being G&T time.
You've cracked it, it is so obvious it's a pity there is no note attached to Norton or McAfee to alert the user.  In Norton I had to set the router as shared and in McAfee to set the range of IPs used by the router as trusted.  Then it all worked.  One might wonder why the first set were automatically trusted and the second were automatically restricted.
Many thanks for your advice.

[#4]

Just seen this post, hence the delay in responding.
It might be that each router provides DHCP addresses on a different subnet.  E.g. one uses 192.168.0.x addresses, the other might be 192.168.1.x.  If all devices use DHCP to get addresses (i.e. don't have static IP addresses set), then the security software (firewalls etc) will think you are now on a new network completely.