Wireless router woes.

[Cold Meat Platter]

Bought a wireless router to connect to my adsl modem/wired router to act as an access point.
Trouble is, when the modem is hooked up to the router by Ethernet there's no connection on either the modem or router. All cables are fine and so is internet access when the PC is connected to the modem. I can connect my PC to the Wireless router and configure it. It's on the same subnet as the modem with a different IP.
The Ethernet activity light on the modem just stays dead when I plug the router into the WAN port or indeed any LAN ports. The WAN port lights up when you connect it back to one of the LAN ports on the router so it's OK.

Modem/router is A XyZel p660r-t1 v3
Wireless router is Tp-Link WR841N
Windows 10

[Sonny_Jim]

If you go LAN to LAN do the link lights come on?

[Uncle TechTip]

Do you need to set the new router to bridged mode, as it might be acting like an adsl modem at the moment, and so expecting just a phone line to be plugged in - when in fact your old router/modem is acting as a router. Confused? I am!

[MojoJojo]

I think you need to configure the Tp-link into access point mode: https://www.tp-link.com/us/user-guides/tl-wr841n_v14/chapter-2-connect-to-the-internet#ug-sub-title-2

[Cold Meat Platter]

New router is just router, no modem. Router is set in access point mode.  If i go LAN to LAN there's nothing. It's as if my modem isn't communicating with the router, it shows 'ethernet disconnected' when, well, it's not.
3 different cables verified working from modem to PC. I had an old router set in bridge mode working fine until it broke recently. Harrumph. New Oculus Quest just arrived...